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.