Van Dijk was 26 when they bought him, and was pretty much a guaranteed instant success as he'd got 3 seasons experience in the league.
Koulibaly will be 28, has only played well in Italy, and does he even speak the language? It's impossible to be the leader at the heart of defence a la Kompany/Van Dijk if you can't communicate with everyone. We could quite feasibly spend 18 months waiting for him to get comfortable here and he'd be on the verge of 30 - or he could never settle in England.
He's a very good player, I'm not sure he'd be a very good transfer for us.