Not stabilizing their outfield mix
It was not until recently that Tommy Edman and Lars Nootbaar began to lock down the Cardinals' outfield for them, but outside of those two, it's hard to see what the plan is for St. Louis.
They've tried Tyler O'Neill, Dylan Carlson, Alec Burleson, and Juan Yepez with mixed results. Brendan Donovan has played well out there, but the Cardinals do not value him as an everyday outfielder as much as they do as a utility man. The club doesn't trust Jordan Walker out there, and rightly so, as he's been arguably the worst defensive outfielder in baseball.
Cody Bellinger is a name the Cardinals were reportedly looking at but were unable to bring into their fold due to the Contreras signing and a lack of a clear role for Bellinger with their outfield glut. There were other names the Cardinals could have gone after as well, but no one wanted to join an outfield that was already way too crowded.
The Cardinals really needed to diversify their assets this offseason and shrink their outfielder pool, while helping to upgrade the roster elsewhere. Instead, they held onto all of their cards, and it has come tumbling down.