Because they were holding him back. He practically had to win the matches against Evolution by himself. Now, with Rollins gone, the other two guys will be exposed and people will be able to appreciate just how great he is.
Seth was too much of a wimp to say the real reason on Friday. Roman is the leader of The Shield and him and Dean will do just fine without him. And who knows, maybe they will find someone who's even better than Seth, and have him join The Shield
