Carson has higher favorables and lower unfavorables than the other candidates. I don't know if that will last, but if the polls are right he stands better with undecideds as of now.
Rubio's an unknown quantity to a lot of people, they can make him into whatever they want, so right now he has a higher ceiling as well.
Poll charts.
Pure conjecture. Trump has been leading the polls for six months approaching 30% in a 16 person field. Trump has more crossover appeal than any of the Reps. His base is solid. No one has yet approached Trump's "ceiling" or floor for that matter. As candidates drop out, where will the support go?
Rubio was part of the Gang of 8 bills pushing the Rubio-Schumer amnesty bill. He is a tool of the Chamber of Commerce pushing increased H-1B visas. If he is the nominee, I certainly will not vote for him.