...
Rich himself just spelled out another reason on the previous page of the thread...
And again, IMPHO, cultural inertia is not a totally invalid reason either. Cultural products (I don't disagree at all with the way Shadowknight defined "art" a few pages ago, so for the purpose of this discussion I'll be considering that an author who lives off his/her work is "a creator of cultural products", not "an artist") don't exist in a vacuum, and the unfortunately-largely-culturally-inert audience (you know, the ones paying your bills in the end) is something that you kinda sorta have to consider.
If you want an honest answer, it would be the exact same excellent story as it is now, except that instead of featuring a bunch of PCs that's instantly recognizable to the average reader as the purely archetypal D&D adventuring group, it would have been featuring a clearly significantly nonstandard PC mix as the protagonists.