Carson is not going to be in the next debate, he seems entirely uninterested in doing anything in this race...from debating to campaigning. I have no idea why he's still running, but he has to be low on cash so he's gone soon.
He's running to sell books, so it's unpredictable when he will drop out. Why do you think he didn't even bother to try to hold onto his Iowa lead and let Trump take it without a fight? Why do you think he's not dropping out when he failed epically in his best state? The guy doesn't want to be president.