Breh, you know why, but there's other reasons too...
Most people aren't interested in stories of evil vs. evil, which is essentially what the media would have to trot out there. Pacquiao simply isn't as popular as Mayweather is, and he's more or less gained a lot of his popularity off of the fact that Mayweather is so hated in the first place, so of course he's not gonna get the bulk of the media attention.
Another thing is Mayweather basically made himself the villain for marketing purposes earlier in his career, Pacquiao never did this. Even without the domestic violence thing people would still be hating on Mayweather. Pacquiao is basically just a more likable person.
There have been Black athletes before who have done some fukked up things off the field/court and people ignored it because they were simply seen as nice guys. And the opposite happens as well (see: Starbury). The American audience is pretty simple, and therefore the American media is extremely basic as well, so this is what you get.