Anthony Downs, writing back in the 1960s, provides a very simple model to explain what has happened in the primary elections in the US this year. The key assumption of the model is that most Americans are inherent moderates and conflict-avoiders — and for a good part of US history, that’s been the case. Even in the American Revolution, the public was divided between Rebels, Tories and a majority who simply wanted the fighting to go someplace else.
In this model, leaders are more extreme than the average follower, and followers are more extreme than independents. The closer one is to the center, the more votes one gets. With his affiliation to the Christian Right, Cruz could be seen as more extreme than Trump, and there is no question that Sanders is more extreme than Clinton. Trump and Clinton are closer to the center and they win.
This model doesn’t explain Kasich’s misfortune. That may be more operational in nature. He just was never seen as a serious candidate by most voters — or was never really seen at all. He disappeared into the wallpaper while Trump and Cruz were throwing bombs at each other.
The model affects the VP choices. McCain was a relative centrist in 2012 and was pulled to the right by his selection of Palin. Something like that can happen again this year.