Because even with Boot Camp, a Mac user would still have to buy their own copy of Windows. Developers can't assume that their target Mac audience is all going to do that, just like they can't assume that everyone has Virtual PC.
Plus, with Apple's limited marketshare, it's not like developers really need an excuse to drop OS X support. For the most part, the developers that aren't committed to the Mac platform already left years ago.
[ April 08, 2006, 05:43 PM: Message edited by: SpaceMonkey ]