They're pretty much different plays. The WR screen, you're trying to get the ball out to the WR as fast as possible, figuring the Dline is pretty much out of the play as they can't get out there fast enough, at least not fast enough to stop the play b4 it starts. THe OLine then pretty much gets out in front & takes on the 2nd & (hopefully) 3rd levels.
On a conventional RB screen, it's slow developing & you've gotta get the DLine to commit upfield to take themselves out of the play. For whatever reason, we're not able to adequately develop these slow "setup" plays.