My take on this is that it could be the Romney campaign realizes that the democrat base is not fired up while most conservatives are pumped to the exploding point. I know I am. Anyone but Obama is how I feel, and I will damned sure vote. They may think that if they come out slinging venom barbs at Obama that it may fire up their base.
Donald Trump and others must have not gotten that memo. They keep warning Romney that Obama is not a nice man and if you don’t take off the gloves you will lose.