In an interview with Screen Rant, showrunner Simon Davis Barry revealed that the original plan was to actually have Shotgun Mary return in Season 2. Unfortunately, Toya Turner left the production, making Mary’s return an impossibility.
“Unfortunately, at the beginning of Season 2’s filming, Toya had to leave the show for personal reasons,” he said. We had to very quickly retool the show to accommodate that. […] It was a very challenging puzzle. But we figured it out in a way that I think honors Shotgun Mary’s character and also sets up the new characters that emerge in Season 2. I’m hoping it plays out well. It’s always awkward when it’s not the plan, right? We did not plan this, but we had to sort of adjust on the fly.”
Source: https://www.distractify.com/p/why-did-toya-turner-leave-warrior-nun