Because they both do the same thing. Keita is even more attack minded than Pogba. If we play a 2 man CM which we do very often it's an extremely unbalanced combination. You'd have to have Pogba sit in behind which is a waste.
We most often play with a 3-man midfield though. Not 2-man. We only played 4-2-3-1 for a couple of months in January-Feb, then reverted to 4-3-3 again. Also people said the same about Pogba-Herrera duo, but worked out alright. If you don't try out these things in training you can't rule them out out of hand. Have you seen the system RBL play and where Keita plays? He's one of the of the deepest 2 CMs they play.