Because he wasn't very suited for our style of play. He is blazing fast, so in a team relying more on counter attacking, he will probably be a great player. But we - especially under Pep - relied almost completely on ball possession and every opponent just parked the bus.
In these situations Costa showed more than enough that he lacks in decision making, precision regardings his passes and crosses and general 'footballing intelligence'.
I don't miss him a single bit, he is sometimes very good, but most of the time pretty frustrating player, at least for ball possession teams against 'park the bus' teams.