Football will never be very popular in the United States for the reasons already outlined above and many of them are just cultural differences. Almost all football players switch to more popular sports where they have a better chance of playing in front of hundreds of thousands of fans and making a LOT of money if they are at the top of their game. There are small, fanatical groups of football fans here, but the sport simply cannot become as popular until advertising money is no longer important to broadcasters (good luck with that). Until that happens, you simply won't see football on American television with any regularity.