But Scheme and Common Lisp (and its dialects) are drastically easier to learn than Haskell.

Furthermore, I think it also has to do with the order in which these courses are taught. When I did a small introduction to FP at my university, a lot of people who had no prior experience in programming picked up Haskell much more easily than those who already had a background in imperative/OO programming.

(Though that strengthens the point - FP is often quite a leap for programmers who have used imperative languages for a long time.)