Maybe its not that the NFL has gotten any better, but the other major sports have gotten worse?
The NBA is in shambles right now in the US. Its foreign appeal is strong, but domestically it is bad. The Lebron James collusion pushed several semi-fans out of the NBA completely, and the owners collapsing in recent negotiations will hurt the sport long term.
College basketball isn't in as bad of shape, but may never climb very high because of the lack of an NBA minimum age. Player turnover is incredible and we are getting one year only out of many star players.
MLB also needs to eliminate guaranteed salaries. Revenue sharing has helped the game somewhat, but there is still a huge disparity.
Why is the NFL popular? The NBA has been reduced to a handful of teams now that players collude to win championships, NCAA Basketball is a one and done affair, MLB still has major revenue disparities.
Only in the NFL can Baltimore be as good as New York...
Slightly off topic, but the revenue disparities in MLB being a direct link to competitive imbalance is a myth. Look at the last 10 World Series Champions.
2012: Giants (Midsize market)
2011: Cardinals (Small market)
2010: Giants (See 2012)
2009: Yankees (Major market)
2008: Phillies (Midsize market)
2007: Red Sox (Major market)
2006: Cardinals (Small market)
2005: White Sox (Major market)
2004: Red Sox (Major market)
2003: Marlins (Small market)
Only 4 of the last 10 World Series have been won by the so-called major market behemoths and their $200 million payrolls (although I don't think the White Sox payroll was that high when they won in 2005). If an organization spends its money wisely, it can put together a contending ballclub.