If I am being charitable it is probably partly to do with how much music is coming out, it is basically impossible to listen to all new DnB releases so it helps that they get categorized so you can find what you want.

If I am being uncharitable, some people just want to differentiate to feel special. Elitism in genres is nothing new either, rock and jazz have their own blend of genre snobbery and I am sure even Classical had people trying to look down on others from adjascent, equal height high horses.