When he drops 8 the OLB in 3-4 have to split the difference from the slot to prevent the quick slant.
So there’s 5 Defenders in the box vs 6-7. The OLB can’t come crashing in on run action because on an RPO half the time they’re the read and have to wait to read pass.
By the time offense have handled the ball off the box defenders are walled off and two yards down the field before the rest of the defense gets there.
This why BYU does better be Baylor and others who use 12 or 22 personnel to establish the run. There’s more in the box