I see what you mean. And my response may be a bit touchy, so don't take offense to it.
But from a marketing standpoint, I can see why 2K has mostly included hip hop vs other genres of music(think NHL games). As much as "we" may want to turn a blind eye to why it's mostly hip hop, the fact of the matter is that the majority of people that play basketball typically come from the same background that enjoys hip hop.
Not to say that including other genres would decrease sales, but more so that including more genres wouldn't necessarily have much of an effect on its diversity because I would assume that consumers love 2K's series for the games they produce, not the soundtrack.