The USA is a relative newcomer when it comes to playing ball sports so they haven't yet 'caught up' with the older countries -mainly European - who have been playing football (ok soccer) for centuries. It'll take a few more centuries before Americans get a true taste for football!
Americans are also quite insular when it comes to their own ball sports - they like to do their own thing and don't much care for 'foreign' imports!
But football is growing. All it needs is a few more role model heroes like Dempsey.Friedel and others and a decent league system to evolve and plenty more sports fans will take to it.