Journals get so many submissions these days, most have introduced pre-screening as a way of reducing the burden on peer-reviewers and editors.
There could be any number of reasons for the actual rejection. It could be to do with scope, technical requirements, readability, impact. I actually discuss this topic in more detail in this blog post:
Zoran is correct Fatma. Not too long ago - all submitted manuscripts would go for review. However, it's a far more competitive 'publish or perish' type of environment where many journals have to 'pre-screen' to manage the sheer volume of manuscript submissions. Most of these journals will send out a standard letter to those that do not make it to review - and often will state that they only accept a certain percentage of submitted manuscripts - to 'let authors down lightly'.
For most journals, there will be an editor and editorial board or committee. Often, the editor is the deciding authority to accept or reject manuscripts. As soon as the paper arrives at the office of the journal, the process of screening begins. Normally, a paper will be read first by the editor or an editorial committee member, who will then decide whether it should go to a referee and whom. Most journals weed out those that are clearly unsuitable in the initial stage itself. A main consideration is whether the article come under the subject area of the particular journal. If the manuscript does not conform to the journal’s requirements as indicated in the “Instruction to authors, it will not be sent to peer reviewers, but rejected.