People are different, each one has his / her own preferences, views, interests. Sometimes what is boring or difficult for one can be interesting for another. Where one person sees difficulty, another person sees a challenge and a chance. The world is colorful and that's fine.
Many reasons can be ascribed for this. It is quite possible that a particular faculty is working on a topic which is of more interest at present. As long as you are contributing to the body of knowledge it is a good topic for indepth study.
When studying for my Masters many years back I put in research topic, The Consequences of Money on Athenian Society. Yes, it was rejected and I was told my premiss was wrong. I refused not to do it. I found subsequently only 3 other people tackling the subject.
My supervisor knew nothing of the subject. Didn't wish to admit it. Instead of placing me elsewhere (honestly I don't think any of the Professors then understood it in its entirety) she refused it. As above, I continued despite that.
The reason of not encouraging the students about specific topic sometimes is the mindset of the teacher because of number of reason that had already been elaborated by friends. In my opinion, its not a positive approach and that idea may have some novelty in it. So, we the teachers must encourage the talented researchers to dig out the knowledge that is still hidden & do the Re Search.