In episode 102 of From Brooklyn to the World, Paulie Malignaggi explains why.
In a nutshell, Paulie says, Conor became a bitch when he lost to Nate Diaz. Not because he tapped out, but because putting himself in a position to tap out was a way out of the fight, without making it look like he wanted out of the fight. Basically, Conor was getting the shit beaten out him and instead of standing up, taking his medicine and trying to win the fight; he got into a grappling match with a black belt in jiu jitsu. He knew he was going to get choked out but thought it was an easier way out than standing up and taking a beating.
Paulie said it was “quitting without quitting.”