Of course US sports lag. We developed our own sports that few others play or nobody else plays. So the audience comes overwhelmingly if not entirely from our own country and nowhere else. Look at soccer which became popular everywhere because its so cheap to play. The only equipment you need is a ball. Compare that to football where you need goalposts for field goals, helmets, pads, etc in addition to the ball.
Combine that with many of our stadiums are full on the same day. Any football Saturday with multiple big teams playing home games, all of their stadiums are full that day. So the Wolverines’ Big House is full on the same day that Bama’s Bryant Denny stadium is full, and the Aggies’ Kyle Field is full, and the Volz’ Neyland Stadium is full, etc (for whichever ones have home games that day). But the whole rest of the world has only enough fandom to fill up only a few 100K stadiums.