Wikipedia is not a reliable source. You might not see a problem with citing a wikipedia page that anyone can edit, but there are very real instances where the page itself has no sources and could potentially be entirely bullshit. I've seen many statements that are actually completely wrong, and have no sources to back them up. When I edited them to be correct, a gaggle of white knights flew in and reversed my changes within seconds, despite the information I put down being factually correct. Perhaps it was because I also didn't source, which is reasonable, but it also shows that if enough people believe it to be true then there's nothing stopping them from monitoring a wikipedia page and make the false change over and over again. It's completely reasonable for teachers to not accept wikipedia as a source, especially at a level above high school.
Besides, Wikipedia is still a great tool, because for information it lists that IS factual, you have the sources laid out for you. You can just find the statement you want to use, go to it's source and then read the source article/book/whatever and cite that on your page. It's infinitely more responsible and technically correct despite you used Wikipedia as a vehicle to get to that source.