Anecdotal, but I've never had a teacher tell me why Wikipedia wasn't a good source. Similarly, I've never had a teacher educate students on how to properly use resources like Wikipedia as a starting point for sources. All my peers and I heard was "Wikipedia is bad, never use it, it's not reliable, don't trust anything from it."
I wish I had been taught why and how earlier, but I had to learn why and how myself.
How were they trying to justify that statement?