Barrett has just never improved defensively and at this point it seems doubtful he ever will. Catcher is just too important a position defensively to have someone who isn't good back there, especially with a young pitching staff. Now maybe the Pads with the pitchers and vets they have, it won't matter. But the Cubs couldn't afford to have him back there any more. And his offense slumped this year as well, though he will hit. That he can do. Bowen is pretty decent, good plate patience. And the young outfield prospect has a a high ceiling.
Trade that makes sense all the way around as SD needs some offense and they have a vet pitching staff. Cubs needed to make a change, pitchers just weren't comfortable with Barrett.