Because the CAS protocol is deprecated? It's arguable simpler and easier to deal with and works really well, but not that well supported and that isn't likely do improve at the protocol won't evolve any future.

We're currently in the process of migrating from CAS to OIDC and so far the server side hasn't been much of an issue, but compared to CAS the clients are a little hit and miss. Authentication always works, but claims are annoying.