Because there are no right guys out there with the exception of Kimbrel and Cora is left in just about the same place Hinch was in last year. "Take Any bullpen and make them effective". Thats optimistic.
Take our own division. Now that Buck has gone off the managerial cliff edge there is one truly interesting, good manager in the whole division, Cash IMO. Cash is followed by Cora who exhibits real possibilities followed by three utter and complete fails since Buck now appears to have lost touch with his team. The bottom three are Buck, Boone and Gibby. Boone only saved from worst Manager in the division by the awful Gibby.
Those percentages are not unusual. They are in fact the norm. Taken across MLB then you would have 6 good managers, 6 more exhibiting signs of and some potential to be a good manager, 6 guys in some transitional phase usually in the wrong direction and twelve utter and complete fails. Cash by the way was awful his first year. Cora is way better than awful in his first season.