As for why, the big three are all candidates fully in the game, fighting and winning elections for a living, and proven vote getters. Not as ideological varieties, but as individual pols. The also-rans are not - they are former minor office holders and one termers, who have backed away from serious fights in the past, and prefer consistency (by their own lights) to winning over the group in front of them.
Pundits hate the way pols win over whoever is in front of them by breezily promising and adapting and playing for the cameras and being drama queens. They think agreement with them, or perfect positioning, or at least the sort of rocklike consistency said pundits prized in (as worse, as)college essayists, "should" matter, more or instead. But they don't. Pols are the way they are for a reason. It works. It is what gobs of real people actually vote for.
Agreed.