Because their fans of other teams, short and simple straight to the point, and point blank. Seriously if every body liked the same team then it would not be any fun. Competition is just wonderful isn't it?
Hate's a pretty harsh term, I prefer dislike. They were always called Americas team, that got old fast. Women dislike the cheerleaders, men enjoy them. I'm from Philly, so Dallas has always been the enemy to us. The team became like a reality show with Romo & Jessica simplebrain. The owner plays a large role in the dislike of the team
The expectation for them so high from the the fans and they lost too many play off plus the management problem for the last several years that make people turn off and hate them. I for one still like them but very seldom to watch their games.