This is what I've heard from a few sponsored riders I've run into in Europe:
1. Parks are bigger and better in the US
2. Consistency and quality of snow are better in the US. Breckenridge (I think) was mentioned as having the most consistent conditions day-in day-out - kind of like going to a skatepark - you always know what you're getting.
3. The volume of snow/powder is bigger in North America. One pro explained this is a big reason behind riders usually going bigger in the US - better conditions for not hurting yourself.
conditions are way more consistent in the US. way more snow and higher quality snow. european resorts have way more vertical typically. many euro resorts have 6000 vertical while the average in the rockies is more like 3000.
north america is the best skiing in the world is the bottom line though.