It's quite simple. They have a HUGE national fan following. Probably as big as any fan base out there. it's simple economics, really.... Many of these teams' fan bases are regional. Cleveland? Cincinnati? New Orleans? Carolina? Atlanta? Tampa Bay? Jacksonville? The list could go on... but many of these teams only have a good following regionally, but NOT nationally. How many Jaguars fans do you know, vs. how many Cowboys fans do you know in your area?
It's all about ratings and how many people you can get to turn it on and watch.
EDIT: I'm not defending the Cowboys. I effing DESPISE them almost as much as the Pats*. Just pointing out why they get the primetime games.