Why are the Cowboys considered America's team?

The Redskins are the most famous team in the NFL. The Redskins has the NFL record for the most points scored in one game,the record for most rushing set by John Riggins,and 3 Super Bowls. You also have to wonder why the Steelers aren't considered America's team if the Steelers have won 6 Super Bowls in all. The Packers were the first team ever established. So why are the Cowboys America's team?