he didn't help himself with the cockiness and arrogance
I've had this discussion with Cowboy fans I know. I think this is the biggest reason that he fell out of favor with the fans.
Players have played poorly before but fans generally give you a break if you own your poor play. If he'd just admit that he played poorly and needed to be better, he'd have a lot more fans in his corner. But instead, his arrogant ass will have a game where he blows multiple assignments and misreads run plays that leads to several big plays, and his response is
"watch the tape. I'm doing my job. Fans don't know our schemes."
Therefore it's no surprise most of the fan base have turned on him.