“Not being seen as a quitter was more important than stopping someone who was so opposed to what you and your party had stood for,” said one Democrat who had been hoping to close the deal.An alternate explanation is that Meek wasn't worried about being seen as a "quitter", he simply couldn't stomach the thought of endorsing Charlie Crist. After all, he may not like Rubio's stand on the issues but Meek is smart enough to know that Crist's "stands" belong to the highest bidder.
Meek is displaying principal. For Mr. Clinton's benefit:
Principle prin·ci·ple [prínssəp'l]
ethical standard: a standard of moral or ethical decision-making