As a person who spent his childhood outside of the US (yes, soccer was king where I come from -- much like almost anywhere outside the US, to be fair), I find it curious that people consider football "too slow". Especially given that it has play clocks, no-huddle strategies, and overall a limited amount of timeouts.
I wonder what this means for baseball (which I find completely unwatchable, based on how much time players take between pitches).
I see the popularity of football as a combination of several factors: (1) the game caters to the macho stereotype; the average American spectator seems to like hard hits and tough guys, (2) anyone can play football in their back yard pretty much at any time, all you need is a ball, (2a) the game makes use of a wide variety of skill sets, meaning football is pretty much for everyone, (3) while the game is QB-centric, good teams are almost invariably more than the sum of their parts, and (4) the game is tactically advanced.
I don't think any other major sports has the full combination of the listed factors. Toss the technological sophistication, tailgating, hard salary cap, and a limited number of games into the mix, and you have a winning formula.