What it boils down to is the games. Very few developers aside from Sony and Nintendo are going to shun the Xbox which means that system will have the most games. The Wii will be shunned because of it's lack of power and the PS3 due to console base (for now)
America's sales are higher than every other category. Partly due to being given first priority. What I find interesting is that it is close to double the "other" category for the Wii and The Xbox but not the PS3 which shows that America is shunning the PS3 just as Japan is shunning the Xbox but not as extremely.
Here's a break down of sales per country and the percentages