Plain and simple: Herman Cain was a disappointment.
Not because of the allegations against him. Not (solely) because he was seemingly having an ongoing relationship with a woman who regularly received financial support from men.
Countless politicians, including Kennedy and Clinton have done worse by actually carrying those "relationships" into office.
But because, by the very act of quitting the race, he signaled that there was likely much more to come.
He left the race in a way that precluded from being considered as a running-mate. He left the race in a way that might discredit the good ideas he proposed and defended.
It was sad to see so much promise evaporate so quickly.