Seems like every QB in the NFL has a number under 20. Every RB is from 20-40, and every WR is in the 80's. What gives?

Its so the refs know who is inelligable and who can or cant go downfirld on a pass play

anyone whose nuber is from 50-79 and 90-99 is inelligable and cannot catch a pass or go downfield on a pass play

Thems the rules. It makes things easier for identification for refs. To ensure no lineman are blocking downfield on a pass or illegally touching the ball etc..