OS X and GNUstep: the only difference

is that you've got that fancy Aqua window manager in OS X.
Besides that, it's the exact same fucking BSD, so why the fuck are so many idiots choosing to develop only for OS X?
Are Apple fanboys really that brainwashed?

Well, there are a variety of cool developer toys (Core *) that aren't in GNUstep yet. But mostly people develop for OS X because unlike GNUstep it actually has users.