There are some common reasons why your professors may not let you use Wikipedia as a source:
Wikipedia is a general encyclopedia.
At the university level, your professors are looking for more than general rudimentary material. General encyclopedias give baseline information, oftentimes it's the type of common knowledge that isn't usually cited. Academic subject-specific encyclopedias will often provide more scholarly and citeable information.
There is often no way of knowing who is editing the entries or what his or her level of expertise is.
You cannot be sure the content is "permanent"
(although you can look at the revision history).
You can't be sure the content meets standards of academic rigor.
One of Wikipedia's main principles is that it strives for a neutral point of view. The problem is that in any knowledge endeavor, much less a collaborative and ad hoc venture like Wikipedia, deciding what's neutral and having something reviewed for this neutral point of view can be controversial and too uncertain to meet academic standards.