I think because the other Thieves World game didn't do well. (Or was it a Rolemaster setting book?)
I haven't seen Thieves World on the bookshelves lately, either, so publishers may not think of it as a viable entity.
Things that haven't been done--actually, if I could think of them, I'd be doing them. But there are some I'm not sure have been overdone.
Ken Hite mentioned the Bone Wars in the American West as paleontologists raced to find and identify fossils; that might make a good mix with a Western slant.
D20 Pinocchio, where players attempt to become real people. (Crossover with D20 Velveteen Rabbit assured.)
D20 Cyrano de Bergerac, where the combat system is extended to include verbal parries and witticisms that do as much damage to one's SOCIAL stat as weapons do to one's Hit Points.