For all intents and purposes both sports simply developed at the same time more or less.

While Association Football was becoming standardized in the 1860's and 1870's. Gridiron football was taking off starting in the 1870's. Then you consider the class of people playing sports in the US, particularly that all major sports found their earliest homes at the Collegiate Level. The Ivy League took a leadership role in developing Gridiron Football, particularly Yale, Harvard, and Princeton. Everyone wanted to be like the elite WASP men of the day to show off how refined and great they were, so naturally everyone was game to play the same sport as they were.

Then you also factor in that US-UK relations remained less than stellar for much of the 1860's and 70's and it is understandable that the sport did not import quickly.