It is because of how defenses are playing us........
The key time to hit a slip screen is when the defense is blitzing and the DB's are way down field. Since teams are not afraid of the deep ball this situation doesn't arise much in the game.
My guess is if we hit a few deep balls they will soften up. So, if we are in a clear passing down, were the defense is going to blitz, with the DB's back. We will probably see it.