It can't be that popular, nobody talks about it like they do about rugger or footy (proper). Do you know anyone who talks about US football?
Also re topic I love the way there's almost as much coverage of US politics as their is UK politics now.
I think part of it is that the BBC's remit is now general entertainment more than public service. Even the news is a form of entertainment today.
The NFL International Series manages to pack London stadia to capacity and has been doing for the decade or more it's been going on. There's an American Football league amongst UK universities that's been running since 2007 and has 75 competing teams. Hardly niche for a sport that hasn't had anything in the way of a grassroots movement here for very long.