Damn it I really didn't want to keep discussing this but you dragged me back in... the silliness of your point demands a response.
region inhabited by indigenous peoples for 10,000+ years is not diverse
Let me remind you, I've been talking not just about the US but western culture in general. Do you believe that Europeans were any less indigenous to Europe, or for a shorter period of time than Asians were in Asia? There's reason to think that Europeans were actually in Europe a bit longer, but it's probably a pretty similar time frame. Mind you, cave paintings have been found in France dating to something like 40,000 years ago. Neanderthals, who Europeans have the most genetic admixture from of any modern group, were in Europe for somewhere around 800,000 years from what I've heard.
Yet, look at the demographics of France, England, Germany, etc today. If your contention is that a people living in an area of the planet for a very long time and being indigenous prevents that area from being racially diverse TODAY, Europe refutes you. Japan, Korea, and China are homogeneous because they choose to remain so. I assure you, there are plenty of people from the third world who would love to live in any of those Asian countries.
Whereas the US (a country of IMMIGRANTS, from all over the world) is dedicated to diversity?
Well first of all, you need to realize that until 1965 the US had an immigration policy specifically designed to keep the nation (vast) majority white. And so it did, in 1960 the US was somewhere around 90% white. Also, the founding fathers were nothing short of racist white supremacists/nationalists, quite openly and blatantly so. Some of them were quite explicit in their intention that the US forever remain white, and they were even picky sometimes about what KIND of white (Anglo-Saxon) - Jefferson even spoke about the US being a nursery from which Anglo-Saxon genes would spread to take over the entire western hemisphere.
So it's pretty hard to argue that the US was never a homogeneous country. It certainly never had the same homogeneity as somewhere like England did, but for being created only a couple of hundred years ago, it came pretty close and stayed that way for a pretty sizable portion of its history.
Now what changed? I think it's pretty obvious that the white majority in the US decided to embrace diversity. the 1965 immigration and naturalization act, affirmative action, giving blacks the vote, abolishing slavery, civil rights, and a black president all happened while whites still held the majority, in most cases while they still held the VAST majority.
Dude, people just came to the US for economic opportunities, and if there were better opportunities elsewhere - they'd leave.
Not entirely true. There were many other motivations particularly in the early life of the nation. People came for economic reasons, yes, but they also came in order to live in a democracy. They came in order to flee religious persecution, and to seize the opportunities a vast unexplored land with rich natural resources presented. They came because they believed in the goals this nation had and the ideals it was founded on. It's true that for most it was a fairly practical decision as you imply, but it wasn't just that.
You're much closer to the truth nowadays though, peoples' motivations for immigrating to the US now tend to be much less idealistic and more practical. Most people in the third world, for instance, would gladly go to any second or first world country, whichever one lets them in, or doesn't stop them from coming in illegally. Usually it's whichever one is most convenient/closest/easiest to get into.
But the last thing I'll say here is that if you want to imply that these economic motivations and such were the only determining factor in who ended up here, you need to realize that lots of people had those motivations and desires prior to 1965 but the majority of them smacked into the wall of our immigration policy, and that trumped their desires. So I don't see how you can say that the US didn't make a decision to embrace diversity.