Why in the world would they ? There are already so many legitimate occasions to bash Romney, so no need to go on hatemongering.
The whole ''Mormons aren't real Christians'' things seems like a legitimate concern, at least for ''real Christians''.
I mean, I think all religions are insane. But y'know, most people don't.
If you take Jesus seriously, and think Romney's belief in whatever Mormons think means he's not a real Christian, well, that could be a legitimate concern.
It also neatly parallels the ''Obama isn't a real Christian'' calls the right entertained last cycle. Except, y'know, they're not based on made up BS, but rather his stated religion.