The thing to keep in mind is that there are a ton of media markets in the US where a good number of people live but who don’t have their own professional team, which leaves them open to various broadcasts around the country. Love them or hate them, the Cowboys are a team that have historically performed decently on the field and a lot of people have watched a lot of in the past, which makes them an attractive option for the NFL to slot into media markets like this (think of places like Portland, Oregon or Albuquerque, New Mexico or Little Rock, Arkansas). If they’re not the clear home team being shown in media markets like Dallas or Austin or San Antonio, and if they’re not the rival visiting team being shown in places like New York, Philadelphia, Washington, San Francisco, or Los Angeles, they’ll likely just be the closest team in general, which means that they’ve got a lot of geographic area that broadcasts they are a part of cover, and therefor gain fans from- when you’re either the most watched or second most watched team in 6 entire states and portions of another 23, it’s fair to say that you’ve got fans everywhere, regardless of who plays locally. Over the last several decades, those fans have moved around the country, started families and gained friends in just about every part of the country at this point and have spread their fandom, to the point where it’s not an uncommon thing to see cowboys fans outside of Texas.