I think wwe have a plan with Rollins, because he is the member of the shield who was put on the singles competition vs Big show, vs Corey Graves (two times). And Ambrose and Reings never was in a singles match. It's 3-0 for Rollins . Joke.
Could it be because he's the best wrestler? You'll notice with the Nexus they had Gabriel and Slater wrestle more than Barrett and Young. Besides, two of those were NXT matches. They don't really count.