Why Humorous Fantasy Isn’t Popular

Dave Langford describes Clench Racing thus:
Up to six can play. The rules are simple: each player takes a different volume of Stephen Donaldson’s blockbuster Chronicles of Thomas Covenant, opens it at random, and leafs feverishly through the text. You win by being first to find the word “clench” (or “clenched”, “clenching”, etc). It’s a fast, furious sport, and a round rarely lasts a full minute.
Clearly, Fandom has a tradition of affectionate mockery of the books it loves. Furthermore, Geeks, the cultural group that includes Fandom, tend to value intelligent wit. It seems odd then that Humorous Fantasy isn’t a massive subgenre.
It’s hard to get facts and figures. An industry insider friend says that Humorous Fantasy’s bestseller/midlist ratio is the same as for other subgenres, it’s just that there’s less of it. Similarly, two authors I know who had humorous fantasy series that petered out both said that the main problem was the size of the market. One of them told me about how at conventions people’s eyes glazed when he talked about his humorous series, but lighted up when he talked about other projects.
Of course, you could argue that Terry Pratchett is so prolific and so very good, that he simply absorbed the subgenre. However, in Heroic Fantasy there’s room for Patrick Rothfuss and Joe Abercromby. George RR Martin may dominate Epic Fantasy, but he has peers. It seems that a typical reader has slots for several favorite authors in a couple of chosen subgenres, but just one slot for Terry Pratchett Humorous Fantasy.
So, I don’t think I’m going out on a limb assuming that Humorous Fantasy isn’t popular compared to other Fantasy subgenres. Why is this?