The Packers do travel well, perhaps the best in the league. The Bills travel well, too. And then there's some teams, like the Raiders, Cowboys, and Steelers, that have a nationwide fan base. Plus, unlike some place like Kansas City, Cleveland, or Pittsburgh, Seattle is a destination city even during the winter.
The root cause of all of this is the availability of tickets. It used to be that the Hawks, and other NFL franchises, would leave just 5% or less of their game tickets available for at large purchases, the balance comprised of season tickets. If you bought season tickets, you got hard copy tickets that had to be handed over in person or snail mailed to a recipient if you wanted to give or sell them to someone else. The result was that the only way you could acquire a ticket to a game was to know someone who had season tickets and make arrangements to transfer the tickets at least several days before the game. Home games were dominated by home team fans, and you very seldom saw an opposing team's fan in the crowd.
Enter the secondary ticket vendors, ie Ticketmaster, Stub Hub, et al, and the advent of electronic tickets that can be transferred instantly. Some people will buy season tickets and sell every one of them, many times at a profit, on the secondary market just so they can maintain the rights to them in case they want to go to a game, if they happen to make the playoffs, etc. It's a problem that afflicts all teams, some more than others. I've gone to road games in Arizona where the crowd is at least 50% Hawks fans.
The only way to fix this is for the team to start winning again, get the mojo back with their fans.