To quote @KLN Kenny's post on this thread from January 29, about the show's decision to kill off Abigail:
"It's a casting and creative decision both, I hear."
Maybe DAYS was trying to figure out what to do about the revolving Abigails problem. And meanwhile they found someone who they think will work out for the show as Stephanie.
Just speculating here, as others have speculated here and elsewhere.
Maybe someone thought well, we've got another heroine (Stephanie) cast, let's just emphasize her for a while, and kill Abigail off for the time being. Not a "replacement" character. Just a redirection of focus for the time being. I think there's room on the canvas for both characters. But just a redirect from Jack's daughter to Steve's daughter for a while.
I'd like all the Johnsons on canvas of course. I'd like Adrienne to be discovered to alive. I'd like all the Johnson cousins (JJ, Abigail, Joey, Stephanie, Sonny, and Sonny's brothers) to all be on canvas. But at least we'll have Sonny and Stephanie for now.