right.. when a QB runs the play is usually fully developed and there's open field. He of course is/was (still not sure if he's lost a step and some elusiveness) one of the best at taking advantage of that.
A RB has to be patient and find the hole, be able to get yards after contact, block, etc etc... there are full time RBs that don't have these skills.
A WR has to catch, know how to run routes and get open, etc etc...