I figure the main reason they wouldn't is so as not to dilute the market of D&D players. Folks that normally don't have a lot of money to spend on RPGs may put their gaming dollar into a Planescape, Al-Qadim, Spelljammer, Kara-Tur, Maztica, or Mystara book by another publisher if they had the choice, instead of spending it on a Wizards of the Coast generic D&D book like Complete Scoundrel, Tome of Magic, or Planar Handbook.
While WotC might still make some money off of it through licensing, they'll make a lot more if folks are just buying books made directly by WotC for its own profit. A lot of folks love certain older settings, and would much rather buy new books for those lines than the latest Generic WotC D&D Complete/Races of/Tome of/environment/creature-type/Monster Manual Book #428.
But they may settle for something WotC instead and pay for it, if the real deal isn't available but the WotC book is kinda close (Planar Handbook instead of a true, full-blown Planescape hardcover/box set, for example). Cityscape may be a halfway decent substitute for a book on Sigil itself, likewise. But if another company was licensed to actually produce Planescape and a Guidebook to Sigil, frex.
So far WotC has only allowed two widely-recognized and liked settings, Ravenloft and Dragonlance, to be supported in 3rd Edition by other publishers. That's a perfectly reasonable bit of licensing to make more D&D players happy and stick with 3rd Edition (thus buying 3E books rather than sticking with their old OD&D/BD&D/1E/2E books).
Then WotC has its own three supported settings; Forgotten Realms has always been a good seller, and Greyhawk is marginally supported by most generic 3E books, a bone thrown to the grognards and folks that don't like the absurdly-high-magic of FR, and Eberron as a newer marketing gimmick and an attempt to draw in younger audiences (and give older players a feeling of new-ness and different-ness to keep them around). WotC doesn't want to spread its marketshare too thin or anything.
Cynical much? Yeah, maybe just a little. But reasonably so.