That's probably because Green Bay is such a small town. Nothing else to do there, so might as well watch football!
Ya, that's part of it. But it's much more than just Green Bay. the team is owned by the city with ties to Brown County. There's a lot of history there, recent success going back 20 years, some popular players, it all adds up.
On a personal note, I lived in Green Bay in the mid 80's and trust me, the only thing worth cheering about were the cheerleaders. Those were some bad teams but still they sold out the stadium in GB and Milwaukee. Through thick and thin the fans have always supported the team. It's really amazing how the NFL and the Packers have grown in the last 30 years or so. I think the last paragraph pretty much sums it up.
"There's no smaller NFL market than Green Bay, which has a metro population of 306,251. However, with 4.4 million Facebook fans and 701,000 Twitter followers, it's clear that there are Packers fans in locations well beyond just the home city. " Personally, I'm surprised
and glad that the Dallas Cowboys didn't rank higher being "America's Team".