Yeah, the difficulty level has generally been suggested as one of the reasons for why they have far fewer fics, but it doesn't explain the lower level of art, vids, etc. Consider how immensely popular Friends was, for example, but it has not spawned a similar level of fanworks.

My own explanation is that comedies have a lot less potential for angst, which is something fans are very into and which fans are more likely to develop around. Also, they used to be much less serialized so plot developments aren't things people would hook into as much.