But what happened to all the good Republican candidates? I'm a long way away from the US, but trying to make sense of it.
The short summary: The primaries are extremely dominated by special interest groups (SIGs), because if they can get a sympathetic candidate the actual election will be a coin flip of who people dislike the least. So what happens is that a lot of moderates get caught in no man's land because the SIGs support their hardline candidate and if you can't get any momentum out the gate the chances of recovering 5-10 states down the line as people realize their favorite won't make it is slim and none. It's hard to find a moderate that many people would be happy with, until it's clear they'd lose and would rather compromise.