I barely even noticed they got replaced. They never got to have any story involvement or character development, just like most of the rest of the bridge crew in Discovery.

These two had the misfortune of being in the grey zone between background and supporting characters. It's fine for a show not to focus on the bridge crew (who's at the helm on the Cerritos?) but Detmer and Owosekun got a raw deal because sometimes it seemed like they were gonna have some character development. Like in season 3 that Detmer had this bizarre beef with Stamets out of nowhere, and that was probably the only episode where she got more than a couple of lines.

I'm not surprised that the actresses chose their other projects (they were probably at the bottom of the totem poll of the show in terms of pay).

I probably wouldn't have noticed they were missing, but in episode 3 Michael introduced Rayner to the crew and said "You'll meet Owosekun, Detmer and the rest of the crew in a little bit", and then later Tilly said "I'll send Commander Owosekun in", off screen of course. Lame.

I guess I shouldn't be surprised, but I didn't realize there were conspiracy theories floating around about them being fired or some nonsense. I figured the actual reason was probably pretty benign.

I missed them. :(

That explains the story they used to explain in universe, but not why they chose not to have them on the final season.