Many different subcultures have evoked the Illuminati for many reasons, it has been featured in comics (Avengers had a pretty good one), books, music and games. And as far as music goes, it's not just rap, many rock/alternative/metal musicians incorporate the symbolism into their overall motif, and others are outspokenly 'against' it.

That being said; money, power and status are often things that groups of people wish to be associated with.

Also, the corporate globalist elites love using musicians as mouthpieces.