it's just genetics dude. that's like asking, "my hairline is shaped X way. Why won't it grow Y way?" it's just the way you're built. also, something i've noticed - chicks don't give a fuck about whether balls "hang low" or not. in fact, if they hang low, they think of them as sagging, and that is something girls DO NOT like. now if they are so tight to your body they look like they're non-existent, that's something else and that just sucks bro.
(note - don't know if you're straight, just assuming.)