Because it is panic time for the Milt campaign again. Santorum is slashing Milts lead in today's polling. Rick is surging at the right time and looks like he is going to make this a real race in AZ. Brewer probably got an envelop full of cash and a few promises to support Milt before Tuesday.
That certainly has been the pattern but if she doesn't want to be shaking her finger in the face of the next president, she will not endorse Romney.
She’ll pick Romney because she needs the Mormon votes for her next and final election, who will desert her in droves if she endorses any non-Mormon.
Yes, there are a lot of Mormons in AZ but they aren't the majority.
Then too McCain has endorsed Mitt so he might have told Brewer to tow the party line.