1)Street focused 'head nodding' beats
2)A large variety of crews playing in the same relative pond
3)Even though we don't want to admit it; dubious lyrical quality...lol.*

*On the last point, take a serious look at some 90's boom bap and you will realize that even on the good to great tracks a predominant amount of those dudes weren't rapping on a high level compared to some of their contemporaries.. As it's been pointed out there isn't that big a difference between 'Hard in the Paint' vs 'Throw Ya Guns'.

The reason is because the best trap/boom bap is really about *perfect* vocal syncopation with the beat (which is why both genre's can be lyrically murky).