Some of it's history, some of it's just general team loyalty. I never heard the Pats having a hard time getting butts in the chair, but New England (Boston really) sports fans are long suffering, they're used to supporting bad teams with no chance of a championship. And a lot of it is the area, Chicago is a general sports town, as long as management doesn't completely screw up anybody there can sell out. Denver is an NFL football town, the Broncos will always sell out. The south east runs strong in football, but not NFL football, that makes for fair weather fans. Arizona has no true sports tradition, we're totally fair weather fans for anything at any level.
The Patriots would only draw well when the team was good. It wasn't until the Parcells-Bledsoe era did they start selling out every game. Even if they do go back to being a bad team I think they've established themselves now so they'll still sell out their home games.