Why do some people not agree with the Big Bang Theory?
It is also, undeniably, incomplete, as it does not explain where the matter came from - just that there was a large explosion.
However, it is incorrect to say it is just a theory, as a theory does not need evidence. Technically, Gravity is just a theory as well.
A Scientific Theory must be created via the Scientific Method.
The primary reasons are therefore that it seems to contradict most religious views of the creation of the universe and that it is not complete.