Because the R's don't have any superstar governors? Arnold is the only possible candidate for that title and he's a Democrat, not constitutionally qualified to be president.
The race is wide open in 2008. I have no idea if an R or a D is going to win.
But why don't we have a decent crop to choose from? Why wasn't the RNC developing WH-worthy Governors 10 years ago? Why did Mehlman and his forebears not develop any?