That's the only thing anyone can say about Romney -- including Romney himself!
As to why Gingrich, Bachmann and Palin weren't invited to speak . . . long memories, maybe? I seem to recall reading that Reagan spoke at the '76 GOP convention and wowed them so much that there was much shaking of heads among the delegates and asking, "Why didn't we nominate him?"
Good point. Romney pales in comparison to any of those three.