You know the answer ro your question, in the spirit in which it was presented. People of different ethnic background and religious persuasions get along much better because, as you said, it is all about individuals not groups.
America has no autonomous provinces for that same reason. Instead, those who cannot be asked to assimilate -- America's Natives -- are simply placed on reservations! I like that! I think the Serbs should do the same with Hungarians and Bosniacs and Albanians! On reservations! American style!. But how did Amereica come to be such a wonderful place (and it is -- now)? After many of it Natives have been forcefully displaced or even ethnically cleansed, my friends. Sure, everything is hunky dory here after 250 years of cleaning house, bloody civil war and an "equal but separate" society of racial nature way into the 20th century. Bravo! I think there is a kind of romanticism, belated or not, flourishing here as well. We are all in awe with the results, but have forgotten what it took to get to this point.
Now imagine what this place would have looked like if every time someone tried to do something someone slapped sanctions on us here? In other words, if every time we tried to do something (and it wasn't always noble either), someone came here to intervene on the world's behalf.
What? You have race riots? Obviously you can't handle race issues! Let's send UN troops to San Francisco and Chicago and appoint a governer from France to administer the areas under 'UN protection.'
After all, Serbia is the only former Yugoslav republic that has some 23 odd nationalities living there. Obviously, they must have some sense of individual rights! No one is revenging Serb death in Kosovo by mugging Albanians in the streets of Belgrade, or God forbid, for using their language as the Albanians are doing to Serbs in Kosovo.
Serbia is closer to Israel in that sense than to the US. It's history. It's so intervowen in the culture that you cannot separate one from the other. After 80 years of communism, the Russians never stopped calling Sunday "Resurrection."
Serbs and Croatians don't fight in America, but don't hang out with each other either. The story changes when one group forces another group from their homes.
The Balkan nations cannot be parceld out every so many decades because one super power or another is unhappy with the arrangement. It only perpetuates the unsettled problem. They too need their time to make everything hunky dory. But no one will leave them alone. It's time to let them arrange their house without any help.