I expect there's more people in the US who care about football than there are in the UK. Most kids play it now, there's tons of travel teams at school level, the MLS is getting better each season, and when the US is at the finals, there's a huge following. Not to mention the effect the prem is having over here, every single prem league game is broadcast over here.
Even at this world cup, the coverage has been second to none, and actually sounds much better than the UK's efforts.
And this is on the Fox network too!
When I was in Australia, it was probably fourth most popular sport behind rugby, Aussie rules and cricket. Sounds like it's getting bigger now according to the above.