I believe because of MLS roster rules. Only other possibility I believe was Politz and he doesn't seem to be a realistic option either. At least Redding had MLS experience. Ndam has been loaned down and recalling those guys isn't an easy thing to do. Plus, sometimes you need to put a guy out there to see what happens when his teammates around him are better. I don't expect Redding will see any more MLS minutes outside of injury.
Ive personally really liked Scarlett but he's only on the RB2 roster.
On a separate note, I really, really hope we can re-sign Parker. That recent interview with him in the Athletic was some quotes from him sounding open to a move over seas after this season and that would be a huge loss. 1, he's an absolute beast. Maybe better than any CB I've watched since I've been a fan of this team and that includes Olave. Secondly, we gave up a lot to get him. That's fine as long as we can bring him back long term, but really rough if he leaves after this season.