Fischer's book is good and well worth reading. But the question isn't just why areas were different in their origins, but why some areas did and others didn't grow closer together later on. Quakers and Puritans hated each other in the 17th century, but such animosity wasn't a factor 200 years later. It's possible to speculate about what would have happened if Puritans had landed in Virginia and Cavaliers in New England. Things would have been quite different.
Massachusans and Alabamans or Mississippians and Vermonters always seem to end up on opposite sides of political questions, but why, at this one point in our history did separation become a live consideration, and not at other times?
Good point. But Ulstermen who moved to those areas were moving into areas dominated by another British culture (or Dutch culture in the Hudson Valley, etc.). I don't know that the Ulstermen formed a majority in these areas. Perhaps they did in some local areas, such as southern Pennsylvania (half of them probably moved South along the Great Wagon Road to the Carolinas and Georgia).
Subsequent generations of the Scotch-Irish probably assimilated somewhat into the dominant local culture, whatever it happened to be. (To the extent that we Scotch-Irish can ever assimilate into another culture.)
Georgians were a blend of cultures also, including debtors from English prisons, Salzburgers from Austria (not in favor of slavery), Irish immigrants, and people moving down from points north, even New England.
Perhaps the Puritans mellowed and became somewhat more tolerant 200 years after branding, whipping, and jailing Quakers and Baptists during the early years of the Massachusetts colony. (Imagine my horror to discover that I had a Puritan ancestor who served in a witch trial.)
The Puritan mindset of self-proclaimed superiority and moral scold persisted for eons -- what was it that used to be said about the Boston Brahmans -- the Lowells speak only to the Cabots, and the Cabots speak only to God?