THIS: 100% this.

"* Shared culture, language, religion? Language, mostly. The Puritans in New England didn't really have much to say to the Quakers in Philadelphia, and both had even less in common with the not-particularly-religious folks down in Virginia. Yes, I guess they all were (or at least professed to be) Christians, but that's not too different than Europe today."

Let me expound upon this further:

The Puritans (not the Pilgrims, who they ended up forcibly annexing into their much larger colony) were by and large the "Roundheads" who won the English Civil War. Ethnically, they were non-gentry, descended from the Anglo-Saxons who lost to the 1066 invasion forces of the Normans.

In Virginia, the settlers were the younger sons of the English gentry, losers of the English Civil War, with lots of titles (if not tons of blood) descending from the Normans. They brought huge swathes of indentured servants with them (the bulk of the white population of Virginia was descended from indentures). They sought to recreate the English countryside in a strictly feudal sense.

In the Deep South, (South Carolina, Georgia, etc) the settlers were descended from the wealthy sugar planters on Barbados. They ran out of land for their sons on Barbados, and the sons came to the Deep South with tons of slaves and a specific goal of recreating the despotic Barbados chattel slavery system and it's huge profits.

Each of these colonies had strict rules on who could enter, and from where.

Pennsylvania was a different story. Founded by Quakers, they pretty much let anybody in, which leads to the Ulster-Scots aka Scots-Irish. Literally hundreds of thousands of Scots who were booted out of northern Ireland or fled the poverty of the Scotland/England borderlands came into Pennsylvania, and immediately went west into the Appalachian mountains, from there spreading themselves and their culture south into western Virginia, western North Carolina, Tennessee, and anywhere else in the US where you picture a rugged landscape that couldn't support plantations. (the places in the US South with rich soil and flat lands were taken over by the slave owners, but the Ulster Scots descendants took all the land that the plantation owners couldn't use, which is why northwest Arkansas is completely different than the rest of the state, or why western North Carolina and western Virginia were filled with Unionist supporters in the US Civil War.

All of these groups HATED each other, and there are still lots of tensions. The Ulster Scots (Appalachian folks) literally fought a rebellion against the US in the early days of the republic due to a financial scam run by Alexander Hamilton and his cronies that is like something out of 1980's Wall Street. (He first colluded with the governor of Pennsylvania to disallow veterans from paying their taxes with IOUs from the US government for war wages, rendering the IOUs worthless, then he and his speculator buddies bought the IOUs for pennies on the dollar. Immediately afterwards, he enacted the US government's plan for buying the IOUs at face value plus interest in hard currency, making his buddies rich. Following this, he leavied a tax on whiskey, which was a valuable medium of exchange in the frontier.)