Mike Tomlin did mention the fact that while Rainey is listed on the depth chart as the primary returner, he's not exactly comfortable with him in that role just yet. If you noticed, Rainey wasn't in any situations where the ball was potentially going to be kicked inside the 10 yd and he would have to field it in a smaller area than if it were being punted from say the opponents 20 yd line. I bet we'll continue to see Antonio Brown field those types of punts and let Rainey continue to work more in on the kickoff return game.
He was itching to come out on those kickoffs though wasn't he? LOL, you could see him just chomping at the bit to try and his hands on the ball and take off.