Toga, while I applaud you for always bringing the international aspect into our discussions, but asking why American schools do not have the "diversity" of countries compared to Europe is like asking why most people living in China are Chinese.
If you compare land area and population, the US is pretty much comparable to all of Europe (not including Russia). The difference is, there are like 20-30 countries in that same area. If you want to be a top business school in Europe, you HAVE to have great students from every country, whereas in the US, they can gather the top quality people with mostly Americans. But even then, 30-40% international students at most top b-schools here is nothing to laugh about, considering what others said, MOST people want to work for companies in the US afterwards. However you want to look at it, the MBA is an American creation and the US, while in lots of trouble right now, is still one of the economic powerhouses in the world.
EDIT: I concede, HEC Paris is pretty well balanced between all the continents, with about 25-30% European, 20% Asian and 20% Indian, and about 20-30% American (North and South).
But like kwam said, even if they're all "Americans", someone like me would not be the same as someone like terry12. Both of us are born and raised "American", but heavily influence by our native cultures. Add that to others who have 1st gen parents from various countries in Europe, Asia, and South America, and add the African American population in, it is quite a "diverse" setting here that is not really replicated anywhere else.
I would agree that most Americans would not be as interested in exploring the "extreme diversities" or be very curious about other countries out there, probably because we're so isolated on our own continent and have been a "superpower" for so many generations that the general psyche is not to really look "out" that much.
I think there's a balance somewhere in the middle between the European model and the US model, and most top schools are really ramping up their international offerings (with a long ways to go), so the trend is good.
With that said, even for a person who likes to experience "extreme diversity" (sounds like an ESPN X-Games type sport), I would not go abroad for my B-school education because I want to do business mainly in the US, somewhere that I'm familiar with, before branching out internationally. If I don't even want to go abroad, and I consider myself pretty "adventurous" for an American, then I doubt others would be that willing.