I think there something about the way we play which makes CB a really difficult role. I'm not talking about the need to pass well, but the defending. With our high line and possession based style, 9 out of ten interventions by our CB will be to prevent or defend against quick counters with insufficient protection. Those are stakes moments in a match.
First of all, that's a big ask of players. Both Laporte and Company sometimes messed up these situations last year's, and they were our best performers.
Secondly, they're hard to scout. Very few teams play like us. You can take CBs who look great at other clubs, but expose them in a high line, back pedalling against Zaha, Traore, Mané etc and many will struggle.