Looking at the countries above the USA... They all mostly have one thing in common, their populace lives almost entirely within a few primary concentrated areas. It is much easier to build out the infrastructure to only support a handful of major cities which certainly helps raise the average.

edit: I want to make it clear that I think the fact that I can get gigabit internet in a town of 300k smack in the center of "flyover country" is absolutely amazing. I think that the USA competes with these other countries despite the size disadvantage is nothing short of astounding.