One thing that has struck me is (outside of the Greeks) how little prominence the Balkan emigrant communities have in American culture. You rarely see the Serbian or Bulgarian communities represented in American culture the way Poles or Italians are. Why didn’t more people from the Balkans leave for the U.S. during the Great Migration period about a century ago?
Around my neck of the woods (Johnstown PA) we did and do have a vibrant Balkan community. I think they just settled in different areas of the country than where you’re looking maybe.
Lots and lots of people from southeast Europe came over after about 1870 during the second wave of mass immigration. But people identified with their smaller local ethnic identities (Bulgarian, Serbian, Croatian, etc.) instead of becoming a large block of Balkan-Americans. If you threw them all together, you’d get a community comparable in size to one of the big groups of “hyphenated Americans” like Germans or Italians, but for various historical reasons they don’t usually identify that way.
Farther to go, fewer direct paths, fewer connections to the US, more negative American attitude toward the people, etc.
When Humphrey Bogart tells you to “go back to Bulgaria” you go. In Casablanca nonetheless.
St. Louis has a large Bosnian population, but more from the early 90’s than turn-of-the century. It is large enough where Bosnia is playing an international friendly here in town in a couple weeks.
There’s a pretty big (relatively) Serbian-American population in California, and strangely enough, there are a bunch of Montenegrins in Alaska, as well as Slovenes in Cleveland.
There’s a very large Hungarian community, at least in some places. I don’t know if you count that as Balkan.
Yeah, I’m around ;)…
As RyJae noted western Pennsylvania got a decent-sized chunk of them. My Serbian great-grandparents ( both my father’s sets ) came from mining/farming regions of Austrian Croatia around the turn of the 20th century and ended up as miners ( coal ) and farmers in PA.
Tons of Serbs, Croats, Bosnians, and Bulgarians around here. I’ve heard an estimate of 250,000 Serbs in the greater Chicago metro area (including northern Indiana), but that was from my friend the Balkan dance troupe director who was trying to explain why the Boban Markovic concert was sold out at a venue that holds a few hundred at most. 
And then, of course, the numbers of Balkan people (even if you count the Greeks) are pretty dwarfed by the numbers of Poles around here.
Anyway, here’s a handy table from the 2010 American Community Survey. I imagine a decent chunk of the people counted as Slavs or Yugoslavian are Serbs. (And a chunk of the 126 million-plus uncategorized should have some south Slavic ancestry.)
Also, who knew there were more people who consider themselves Albanian than Serb in the U.S.? Not I!
A (Serbian) friend of mine pointed out that Chicago would be something like the second or third largest Serbian city based on population.
Just like the last two decades back home, eh?