Because life is always messier than it seems.
What you are saying would be the equivalent of saying that "New Mexico" should belong to "Mexico" (and all the non-Hispanic Americans should get out), just because the state's name contains "Mexico". Superficialities can be deceiving.
Serbs have been in Croatia twice as long as America has been a country. Serbs had been in Bosnia almost a thousand years before Islam even showed up in the neighborhood and created what you're now calling "a Bosnian".
But, let's say that, even given their history, Serbs were willing to fit into that little cookie-cutter fantasy because they weren't aggressive and dominant enough to demand that wherever they lived became "Serbia". What would that do for America, Europe and the world?
A Muslim corridor, which in my opinion has always been the Islamic plan, would ultimately be created all the way from Turkey in the Bosphorus (East) to the Albanian Coast (West) and straight up North to within 60 miles of the border of Hungary. It could and would endanger all the countries of the former Yugoslavia, including Macedonia, Serbia, Montenegro, Croatia and Slovenia even further, even though Croats & Slovenians act like they aren't worried. All it takes is a good look at a map and a knowledge of where Muslims are compact minorities in those countries, to figure it out.
Like I said, reality is always messier than it seems.
great post