Alternatively, there's simply no evolution there. No progress of ideas. No advancements of theories in music. A broken record playing a stagnant tune, but it plays it because that's what the core demographic wants.

There is advancement in the genre, but it tends to quickly leave the genre, since the genre is analogous to a spoken word album with a BGM. Anything interesting or creative, finds itself quickly ceasing to be rap. There's dozens of different "metals", which themselves are an evolution of "hard rock", which is itself an evolution of "rock and roll". Likewise...

In a way, EDM and Dubstep are evolutions of rap. You're just dropping the spoken word part of the music, and only focusing on the beat/rhythm/music part of it, and letting them develop being freed from needing to play second fiddle to a vocal track.

But the core demographic want basic repeating beats with occasional bass boosts (see? EDM and dubstep's origins there) with some guy loudly and tunelessly swearing about how he likes to commit petty crimes or at least antisocial behavior. And so the "core" of the genre doesn't need to move, because the audience laps it up every time.