because yaoi blend of emotion and beauty, involving two people whose love is looked down upon. The art tries to show that all love is magical.
Generally neither of the MCD are complete twits unlike in Shoujo.
Because the straight version is the so called 'hentai' and no women want to read that because the women in it are just used and abused and it's not hot at all.
See also: why is gay porn so popular with women? Same reason, in straight porn women are just used and sexualized and it's just easier to just enjoy two guys which doesn't damage your selfesteem.
So Y aren't black characters thrown into everything, they could use an artistic medium that exposes there "inner beauty"
Y is Yaoi so popular???