Ok, so the more verbose explaination:
It's not racist to review or not review anything. Even rap. It's not racist to like or not like any music. Even rap. It's a copout to say it, it's typical liberal guilt. People review things that they think their readership is interested in or that they themselves are interested in. If you want there to be more reviews of rap for audiophile readers, find some, and review it. Honestly, there is discussion of rap even on headfi, it's just not as common as many other genres. Why does deathmetal get discussed? Because the people that like it are nerds who spend all their time on forums, and talk about it.
Tyll said it took him a long time to find a well recorded rap piece to put on his sampler, and he ended up with MC 900 Ft Jesus. It's just not something people do anything interesting with in the recordings, generally speaking. I like a lot of rap, but I'm not excited about it generally, because its' the same stuff I've been listening to for 10 or more years. I haven't discovered new rap in ages.