This is my take on it. Moron evolved to be a more general term associated with anyone acting foolish. It evolved to the point that nobody really associates it as a word targeted directly at intellectualy disabled folks. The r-word was always more commonly understood as "the word you would say to a disabled person if you wanted to hurt them the most". Because of this people who use it weren't/aren't given the benefit of the doubt that they're just being hyperbolic and not actually a bigot directly to disabled people. Why did one evolve to be more hurtful than the other? I'm not sure. There's probably a parallel universe out there where the reverse scenario exists.
Edit: nvm, disregard. just realized this post is 10d old and people already articulated what I intended much better than me.