Just about every non-Cowboys NFL fan knows at least one lifelong Cowboys fan personally whether it be family member, friend, co-worker, etc. The same can't be said for any other team outside their home market.
The sad thing is that many Cowboys fans hate their own team worse than many fans of other teams. They may only hate one or two aspects of the team, but they'll hate on the team and gloat after every loss more so than the fans of our most hated rivals, so it's no wonder the Cowboys are so hated.