Because I said so.
(Just kidding.)
You can't cite Wikipedia as a source in a research paper because too many Wikipedia pages are untrustworthy. For example, I have recently come across pages that contain information that is plagiarized. For another example, how about a page containing information that's just plain wrong?
That being said, if you are totally ignorant on a topic, it's OK to look at a Wikipedia page, especially its links to more reliable resources (if you google, you're going to get Wikipedia as a top result anyway).
