Some of the issues with building fan loyalty in Atlanta has been the lack of consistent success until recently. The Falcons never had consecutive winning seasons until 2008 and 2009. From 1966 onward until then, every winning season was preceded by and followed by a losing season. There were no dynasties, even along the lines of several consecutive division titles or wildcard bids. Even when the went 14-2 and made the Super Bowl in 1998, they went 7-9 in 1997 and 5-11 in 1999. With all of the other things handicapping the Falcons, the lack of success certainly hasn't helped.
That's another difference. The Eagles had long stretches where they were awful yet they still had an avid fan base. There was one stretch where they were bad from 1962 to 1978 or 79. Then they had 1 good period. Then many more years of losing. Then came Buddy who couldn't win playoff games. After several years of BS, we had our silver years with Andy Reid. Couldn't win the big one.