Here's the real theory on it, and believe me I can speak from experience. Friday's Hardcore party about 40 people, Saturdays Hip Hop over 200 people!!
So anyway, the reason??
Plainly and simply image!! In case you guys haven't noticed, kids have classic sheep mentality. They only ever do what the others do and nobody dares to move from the trend. At the moment the whole gangster black American imagery is huge with all the music videos and you only have to look at the top 40 to see what videos the kids are watching. Not only that but kids into hip hop tend to travel in packs (security in numbers etc.) and hence the crowds get much bigger much quicker.
The reality is you could be playing Billy Ray Cyrus and as long as the cool kids thought it was "dope" then the place would go off.