..., but why is there no "I want a black / Indian / off-white Doctor" cry from the Internet? Is there an in-universe explanation for only white Doctors?

The discussion I read somewhere is that Moffat won't do a female Doctor, because DW has become to much of a British institution and thus an iconic image. But if DW has shown anything (in terms of interesting background, side and supporting characters) is that Britain has become a multi-coloured society. I think a Black (coloured) Doctor won't be a bad choice.