Football is by far the most played sport on the planet.
The appeal is, I think, in its simplicity. You don't need a lot of kit to play it - just a patch of ground, something to serve as goalposts, and a ball.
The rules are simple. Even the offside rule is simple.
Gross generalisation follows.
It seems to me that the American public doesn't like sports that (a) don't have double-figure scores, (b) weren't invented there (golf and tennis are notable exceptions) and (c) don't break into quarters.