Because djs like to mix songs continuously in a way that it sounds like one song is flowing seamlessly into the next. It only takes subtle differences before a transition throws you right out of the moment. It’s possible to mix these subgenres together but you have to do a lot of work to find songs in different genres that sound similar enough to make a smooth transition.

Djs don’t buy all one sub genre, generally, but they do tend to focus on a few of them and that’s why the labels and record stores try and cater to what djs are looking for.