There are lots of stories and myth circulating but none are true. In their book, "Is There No Nobel Prize in Mathematics?" by Lars Garding and Lars Hormander, the authors dispel all myths and state quite clearly: "The true answer to the question is that, for natural reasons, the thought of a prize in mathematics never entered Nobel's mind."

"Nobel's final will of 1895 bequeathed $9,OOO,OOO for a foundation whose income would support five annual prizes in physics, chemistry, medicine-physiology, literature, and peace. Four of the original five prizes were in fields which were close to Nobel's own interests, medicine being the exception."

This same question, "Why Isn't There a Nobel Prize in Mathematics?", has been answered by Peter Ross, professor of mathematics at Santa Clara University:

http://mathforum.org/social/articles/ross.html

"The more mundane reasoning behind him not selecting mathematics for a Nobel prize is thought to be simply that Nobel wasn’t very interested in the subject and didn’t grasp the practical benefits to the world of advanced mathematics. The Nobel Prizes were created as awards for people who made the greatest contributions to mankind in subjects that interested Nobel. So it would seem Nobel just didn’t see the benefit in providing one for mathematics, a subject he didn’t care for anyways, and, rather, he stuck to ones he understood well the benefits to mankind and subjects he was more interested in: Physics, Chemistry, Medicine, Literature, and Peace.

The above is the generally accepted theory based on historical records, however, an alternative theory is sometimes suggested, though it is pure speculation. At the time, there already existed a major mathematical award that was actually established at the behest of Mittag-Leffler himself. Mittag-Leffler persuaded King Oscar II to create an endowment prize for various mathematicians throughout Europe. Thus, Nobel may have simply not wanted to try and compete with that established award with his own. Rather, he focused his funds on fields that interested him that didn’t already have prestigious awards attached."

http://www.todayifoundout.com/index.php/2010/10/the-reason-there-is-no-nobel-prize-for-mathematics-had-nothing-to-do-with-any-wifemistress-of-alfred-nobel/

Nobel Roze is honored for Physics, chemistry, Medicine, economics, Peace Initiatives and Literature etc., However mathematics is prevalent and intertwined in the above fields..

I think that the proper question is: Why Alfred Nobel did not want to include a Math prize?

Somebody suspected that a mathematician had an affair with the Nobel's wife, or perhaps maths were not an Nobel's skill.

The nature of maths cannot be a reason. Literature is not a science and there is a prize for it. In any case, it does not matter. For instance, Einstein is great by his scientific achievements by no means by his awards.

According to one link (with detailed research), this is the conclusion made:

"Nobel, an inventor and industrialist, did not create a prize in mathematics simply because he was not particularly interested in mathematics or theoretical science. His will speaks of prizes for those ``inventions or discoveries'' of greatest practical benefit to mankind."

https://cs.uwaterloo.ca/~alopez-o/math-faq/node50.html

There are lots of stories and myth circulating but none are true. In their book, "Is There No Nobel Prize in Mathematics?" by Lars Garding and Lars Hormander, the authors dispel all myths and state quite clearly: "The true answer to the question is that, for natural reasons, the thought of a prize in mathematics never entered Nobel's mind."

"Nobel's final will of 1895 bequeathed $9,OOO,OOO for a foundation whose income would support five annual prizes in physics, chemistry, medicine-physiology, literature, and peace. Four of the original five prizes were in fields which were close to Nobel's own interests, medicine being the exception."

This same question, "Why Isn't There a Nobel Prize in Mathematics?", has been answered by Peter Ross, professor of mathematics at Santa Clara University:

http://mathforum.org/social/articles/ross.html

"The more mundane reasoning behind him not selecting mathematics for a Nobel prize is thought to be simply that Nobel wasn’t very interested in the subject and didn’t grasp the practical benefits to the world of advanced mathematics. The Nobel Prizes were created as awards for people who made the greatest contributions to mankind in subjects that interested Nobel. So it would seem Nobel just didn’t see the benefit in providing one for mathematics, a subject he didn’t care for anyways, and, rather, he stuck to ones he understood well the benefits to mankind and subjects he was more interested in: Physics, Chemistry, Medicine, Literature, and Peace.

The above is the generally accepted theory based on historical records, however, an alternative theory is sometimes suggested, though it is pure speculation. At the time, there already existed a major mathematical award that was actually established at the behest of Mittag-Leffler himself. Mittag-Leffler persuaded King Oscar II to create an endowment prize for various mathematicians throughout Europe. Thus, Nobel may have simply not wanted to try and compete with that established award with his own. Rather, he focused his funds on fields that interested him that didn’t already have prestigious awards attached."

http://www.todayifoundout.com/index.php/2010/10/the-reason-there-is-no-nobel-prize-for-mathematics-had-nothing-to-do-with-any-wifemistress-of-alfred-nobel/

 Kindly read Daven Hiske's article dispelling the myth :

Article title: (see link for content)

The Reason There is No “Nobel Prize for Mathematics” Had Nothing to Do With Any Wife/Mistress of Alfred Nobel

http://www.todayifoundout.com/index.php/2010/10/the-reason-there-is-no-nobel-prize-for-mathematics-had-nothing-to-do-with-any-wifemistress-of-alfred-nobel/

There is no NOBEL PRIZE to Mathematician despite the fact that Mathematics is the mother of all sciences. Whatever the reason is? I think it is not acceptable.

There is the Field's Medal for Mathematicians.

When,' One is of everybody, means he is of nobody'. May implies to Mathematics.

I do not think his decision not to include mathematics  in his will was based on his dislike to mathematics. He was a chemist and engineer and therefore he knew the vitality of mathematics to humans. But I suspect his decision was much based on his understanding of  mathematics as a natural activity of humans like breathing and when he put fields in his will that should be supported, they were the ones which bring physical influence on society. He said  his fortune be used to those who made "greatest benefit on mankind" in physics, chemistry, peace, physiology or medicine, and literature". Mathematics does not care whether a certain prize is given to it or not, how popular it may be. There are more than 10 prizes awarded to the galaxy of mathematics which are quite as remarkable and valuable to it as Nobel prize is  to physics, chemistry, peace, physiology or medicine, and literature. The Fields and Abel's medals are among them. 

Good question. Lots of interesting links. Yes - no prize in the field of mathematics. But to say that among the winners of the Nobel Prize no mathematicians - is not right. Just such people received the Nobel Prize in other fields of science. Thus emphasized the role of mathematics to other sciences. Therefore, math is not forgotten.

As @Abedallah stated, The Fields Medal, "officially known as International Medal for Outstanding Discoveries in Mathematics, is a prize awarded to two, three, or four mathematicians not over 40 years of age at each International Congress of the International Mathematical Union (IMU), a meeting that takes place every four years. The Fields Medal is often viewed as the greatest honour a mathematician can receive." It is NOBEL for Mathematics! Funny, Mathematics is not a field for Nobel prize, but mother of science!

http://en.wikipedia.org/wiki/Fields_Medal

Nice saying dear Ljubomir. "Mathematics is not a field for Nobel prize, but mother of science".

Probably Nobel had in mind the applied kind of sciences, because they can improve the life of all people by an explicit way, while Mathematics can do the same thing, but in the long term...

Even though there is no Nobel Prize for mathematics, mathematicians still often win it in other categories. The most prominent such mathematicians are Bertrand Russell, a prominent philosopher as well as a logician, who was awarded the 1950 Nobel in Literature and John Nash who was awarded the 1994 Nobel in Economics for his study of equilibria in non-zero sum games - Note that a sixth category, Economic Sciences, was added in 1969.

The 2007 Nobel Prize for Economics has been won by Leonid Hurwicz from the University of Minnesota, Eric Maskin from the Institute for Advanced Study, Princeton, and Roger Myerson, from the University of Chicago, for their work spanning 50 years in a branch of game theory called Mechanism Design Theory. Hurwicz, at 90, is the oldest person to receive the Nobel Prize.

http://plus.maths.org/content/2007-nobel-prizes-won-mathematicians

@Issam link is good one. Following some links in this article, I have found another good article Nobel mathematics by Rachel Thomas, Helen Joyce."The choice of the 2002 Nobel laureates, announced in early October, shows the vital role mathematics plays in all the sciences, illustrated by the fact that the work of these researchers would not have been possible without mathematics."

http://plus.maths.org/content/nobel-mathematics?src=aop

Dear Mehdi Rajabi Asadabadi , Your answer is very nice Haaaa

Mathematicians are honorable............Nobel prize is not actually able for mathematicians..

@Dear ALL,

You can see the subject from different perspectives, Also find some more area for the NOBEL PRIZE (attached linked).

https://www.researchgate.net/post/What_new_category_would_you_suggest_for_the_Nobel_Prize

Manjul Bhargava, recipient of Fields Medal 2014.

The Fields Medal is widely considered to be the most prestigious award in mathematics, and the 2014 Fields Medalists were just announced.

The Fields Medal is given to between two and four mathematicians under the age of 40 at the International Congress of Mathematicians, held once every four years. According to the International Mathematical Union, the body that awards the Fields Medal, the award intends "to recognize outstanding mathematical achievement for existing work and for the promise of future achievement."

http://www.businessinsider.in/These-Four-People-Just-Won-The-Most-Prestigious-Award-In-Mathematics/articleshow/40133407.cms