Same reason why he knows he can't rely on a primary passer at the QB position. But then you'll say, "But Nix can run." But then I'll say, most folks are acting like Joey can't hit the broadside of a barn.
We'll see what happens in two weeks!
I thought this same thing while watching him on A-Day. Just doesn't make sense to be handing the ball to him anywhere outside of the 5 yard line or a short yardage situation when you want a big body RB.