Because this is a conservative site. If Romney would just become a democrat the ad wouldn’t be there. Maybe Lindsey Graham could take his place of honor.
“True conservatives” cannot accept that moderate Republicans exist. In fact many of your moderates also hold some conservative views, but this is too complex for the “everything must be black-and-white” and in simplistic terms crowd. Simple thinking is what got Obama elected.
The infighting among Republicans, conservative vs. moderates, is nothing new. It’s a bloody life-and-death struggle that allows democrats to remain in power.
For myself I look at our current scene as American vs. anti-American. I’m on the side of America and so is Mitt Romney. I have no idea what his political ambitions are at this time, but the gratuitous attacks on him look like pure bullying to me and, in fact, make him look more attractive.