He's just overall sort of an unassuming, restrained person (regardless of what you think of his views, he does strike me as a reasonable kind of person despite my disapproval) and before we even talk policy those guys don't generally do very well in American politics as they make very easy targets for loud guys like Trump. He's also the brother of a guy commonly viewed as a bad/idiotic president; because of the family history with the position he'd normally be pressured to use it to his advantage but his brother's terms made it actually disadvantageous to do so. The guy was essentially left with no real substance to his campaign as a result, so this was only a matter of time.

A bit of both. He's completely incompetent and uncharismatic, his ideas are outdated, and his only ties are to a president who caused a completely pointless war. He had no chance.