Nate Silver thinks his sane moderate conservatism makes him a no-no for the reactionary populism now defining the GOP. The core problem:

Expressing support for Barack Obama’s 2009 stimulus package, endorsing an individual mandate for health insurance, and securing Utah’s participation in a regional cap-and-trade program. Mr. Hunstman also holds a moderate position on gay marriage, having endorsed civil unions for same-sex couples. And he served as President Obama’s ambassador to China.

This would make him an accomplished pragmatic conservative in any other Western country. In America, in the GOP today, he's a commie.