He's the darling of the ideological puritans, who'll scuttle their own ship rather than have it lead to victory by a captain who doesn't quite meet their purity standards. I believe it's called cutting off one's nose to spite one's face, but hey, the 100 or so people who actually vote for Hunter in the primaries can hold their heads high, knowing they didn't sully their purity.
And note, this is from a guy who rather likes Hunter, but also lives in the real world.