The far right wing candidates are too weak or too dumb to challenge Romney.
The fact that someone like Perry is continuing with a doomed campaign that divides the far right vote means that a plausible alternative candidacy is not able to coalesce. Divide and rule. The sense that Romney is bound to be the nominee is achieving the desired momentum, even though he has around 25% support. But 25% beats Paul's 20% etc.
Romney wins.
But he's an awful candidate. I feel sorry for the Republican party if he's the best they've got.