I am not too surprised at France and UK but last time I checked Spain was in the top 3. Not saying Spain is not a beautiful country but I just didn't imagine so many people visited it.
The US has a huge population and the visa process like many people have said already is quite complicated
North America is more isolated from the rest of the world than Europe, which is smack dab in the middle of all of the continents (sans Oceania and Antarctica).
It's much easier to travel around europe than it is to travel to the US. I'd bet a good amount of those tourists to the france and the uk are from other european countries.
Because the US is having its Britney Jean era due to the fact the country is a ****ing mess and who wants to visit that mess of a country when you could go visit Europe where things are much better?