Why would Hollywood want to piss off a bunch of people just to push their own political agenda? That is what artsy movies are for.
The question isn't only why can't Bond be black but why should he? Bond has always been white and I can't think of any good reason to change that. You could make the same argument about a gay Bond. Why couldn't Bond be gay?
I think the answer is because he is a character from a story who was neither black nor gay. If you want a black or gay super spy fine, he can even be a 00 he just can't be Bond because Bond was neither. Bond is a culture icon in both England and the US and doesn't need to re-envisioned.