I think it's the "event" factor.
Watching football on the weekend (or Monday night) has been an american tradition for decades. It's something that defines the weekend for many men, something they shape their schedule around. They do it with friends, or with family, they talk about the games during and after and at work the next day, and the interest is pretty much uniform across the country and in every age group and racial demographic.
That's what makes football a viable "live" event. No other sport has captured a time slot or worked its way into the American psyche and schedule so fully.
Football also has a game flow that lends itself to the live TV experience really well. With a basketball game, you're often sort of expecting that the score will ebb and flow and you don't want to take anything too seriously until the second half (unless one team starts blowing the other out.....but then the second half becomes boring). Whereas in football, it feels like there's so much riding on every drive that it keeps the casual fan's real-time attention better.
Also, I'm not sure if this is a factor or not, but football lends itself to big screen TVs more than anything. Watching on your phone or computer doesn't do it, for years now people have been buying huge TVs specifically to watch football on them. They're not going to give that up easily.
