They're already regulars. Also starring means regular. There's only so much room in the opening credits.
Ah, I see. I hope now that three spots in the opening credits are free McBride is added, she deserves it the most because she has been with the show since season 1 and because I really like her.
I don't get why Danai Gurrera made it into the opening credits right away, she was a new addition, is not who I'd call a well known actress and her role in the season wasn't that important.
Were they considering to kill Carol? Maybe her "death" in the first half along with T-Dog was originally meant to be real and they changed their mind at the last minute. They gave the spots in the opening to actors they knew would stick around until the finale, not the potential death candidates like Carol.
I'm also happy to read that the walkers will be a bigger threat in season 4, looking back at the second half of season 3 it really seemed they were no longer a big deal most of the time, more like really ugly garden gnomes in the prison front yard.