Couple reasons. The main one being that while B&M build great coasters, they aren't known for their flexibility.
(Example: Hulk at IoA. Universal wanted a launched coaster with customized trains. B&M refused to build a launched coaster. So, to keep B&M in the deal, since they were building Dragons, they let B&M design and fabricate all of Hulk except for the launch system which had to be farmed out to another contractor to design, test and install.
Same with the trains. B&M delivered their standard trains and Uni built the fiberglass modifications.)
Disney (with few exceptions) requires a coaster company to work very closely with WDI on every step of design and construction of their coasters. Most coaster compaines don't like to give that kind of power to the buyer.
(Example: Vekoma and WDI were jointly involved in all phases of the coaster development for Everest.)
Note: Don't look for another Intamin for Disney, either. Disney ed them off by making too many modifications to the ride system on Screamin'. So much so that Intamin doesn't service it or claim it as one of their coasters any more.
Another reason for no B&Ms at Disney: sound. Have you heard a B&M? Their trademark "roar" wouldn't really work in a heavilly themed environment.