The fact that there have been regular season games in London since 2007, with a 90,000 seater stadium selling out incredibly quickly often in under an hour, has definitely helped the sport popularity. Another important factor has been that the TV coverage has increased with numerous matches on free to air TV channels as well as subscription ones. And the sport has been increasingly popularity for UK gamblers to place bets on. Plus away from the glitz and glamour of the NFL, there have been a rising number of amateur american football teams in the UK as well.
In the US, does the fact that the season is so short (just 5 months) compared to the MLB, NBA and NHL ones help it's popularity? Often less is more, and there is a much longer period to miss the sport and crave it's return during the off-season.
In the NFL every match matters, with teams playing a 16 game regular season and a maximum of 19 to win the Superbowl. And there is a healthy amount of parity and teams that can compete for the superbowl.
But in the MLB, NBA and NHL the seasons can drag on and on with so many useless regular season games when the players are going through the motions and trying to save energy for the playoffs. The NBA is incredibly predictable with a terrible lack of parity, and for many younger people I guess that baseball games are too long, slow and low-octane.
