Its just a brilliant game. I think the London franchise idea and the others in the pipeline now and over the years are a distraction - don't see how it improves the game, could tip the entire structure off-balance.
Because the NFL is a business. Viewership in the U.S. is declining slightly and so are the numbers of young boys entering youth leagues. They have already saturated the U.S. market as deep as it is ever going to be.
There is a lot of competition for people's time...specifically computer games and soccer (football).
There has also been a lot of negative publicity about injuries, especially concussions.
If they expand the league to London, Moscow, Berlin, Vancouver etc....they would have new audience potential. More young people playing the game in youth leagues that play in college and move to the NFL. More teams, more broadcast revenues, more profit sharing, more money...
Believe it or not, the NFL is a non-profit organization. The league is non-profit but the teams are not. They pay taxes. But the teams generate billions of dollars every year and the players make millions...sometimes tens of millions of dollars a year with endorsements.
It's all about the money.