How did that change, you may ask?  Well, it all comes down to the English Civil War.  (No, seriously, it does!)  The English Civil War was a series of conflicts in the mid 1600s (so about the time that the first colonies in what is now the US were getting settled) where the middle class and the nobles fought bitterly about what sort of government England was going to have and what sort of religion.  The middle class were in favor of Parliament controlling things, and in favor of sober, stripped-down, moralistic religion (i.e. Puritanism) and the nobles were in favor of the King controlling everything and lots of bells and smells and no strict moral guidance to trouble them.  And at various times, England wasn't really safe for one group or the other depending on who was winning right then, so people left in droves.  The middle-class Puritans settled in the north, and the aristocratic Cavaliers settled in the South.  Now, other groups came later but those first settlers were really, really important because they were the ones who established governments and cultural systems that later groups then had to adapt to.  And each group took the bit of 17th Century English society they liked and tried to recreate JUST THAT SEGMENT of it, with all the rest gone.  Middle-class Puritans wanted everything to be middle class farmers and businessmen.  They purposefully excluded both rich and poor alike from entering their colonies, and set up laws and such that benefited the middle class above all.

The Cavaliers were just the opposite.  They came from the aristocracy and the wealthy, and they wholeheartedly believed in its values--many of them had been forced to flee from their homes because of their commitment to the "ideals" that certain people are just better than others, with an inherent right to rule and control society.  Or they were younger sons who wouldn't inherit the family estate and, for the first time, had some options besides "become a priest/lawyer/soldier."  They could come to America and have an estate here!  THEY wanted to be the lords of Great Estates like their older brothers back in England had, with lots of servants and other peons to work the house and the fields, while they sat in their drawing rooms and played cards and threw parties.  (Nice work if you can get it.)  And they really DID NOT WANT the sort of influential merchant/middle class that had been so troublesome back in England.  So they set things up to benefit the super-rich elite at the expense of, well, everyone else, and started shipping over slaves and indentured servants in job lots.

If your high school US History class mentioned indentured servants, it probably lied to you by trying to tell you something along the lines of indentured servitude being really different from slavery.  But the truth is, they were really, really similar.  Indentured servants were poor Englishman working for years to pay off the cost of their passage to America, and would be free after a period of some years.  Slaves were Africans or Native Americans (to start with, at least, although it quickly shifted to only Africans) who had been captured in Africa and sold to Americans.  And they had no such time limit.  But it wasn't unusual for African slaves to get freed eventually as a "reward" for service, and as for how they were treated, well, the elite who owned both groups didn't really make much distinction between them.  And life for them (for anyone in the Southern colonies who wasn't part of the elite) sucked so bad, you guys.  SO, SO BAD.  Even once the slaves and indentured servants got freed, life really sucked, because the whole point of the society was to separate the elite from the peons and make sure everyone stayed in their place.

And then, in 1676, exactly one century before the Revolutionary War, it happened.  Bacon's Revolt.

You've probably never heard of it, and truthfully you don't need to know much about it.  There was a lot of complicated stuff about hating Native Americans and internal English politics, but that's not important for our purposes.  What IS important about it is this: it scared the southern elite out of their ever-loving minds.  See, while Bacon was off doing HIS part of it, the slaves and indentured servants (and all the poor people who had been freed but used to be slaves or indentured servants) got together and decided, oh, hai, we have a common enemy, those aristocratic dudes who are oppressing us, let's get together and BURN THEIR HOMES AND CITIES.  And then, after the revolt in Virginia was put down IT SPREAD TO MARYLAND.  And abruptly those Cavaliers realized OH CRAP, THERE'S A LOT MORE OF THEM THEN US, and if we don't DO SOMETHING they will kill us all and destroy our aristocrat's utopia!

What they did about it was create the slave codes and the whole idea of "white" and "black" in an attempt to play poor whites and blacks off against each other so that the two groups would be so busy fighting each other they wouldn't have time to go after the elite.  Which, sadly, has worked.  They eliminated indentured servitude and hardened the rules for slavery, so that they could go to the poor "white" people and say "look, our skin color is the same, our cultural heritage is the same, you are JUST LIKE US, and not like those horrible "black" people, and your life is so much better than theirs is* and let's all be WHITE together and if you help us oppress and control those slaves we'll only oppress you a little."  And instead of saying "Screw you, we're going to join with the slaves and send you all packing back to England so there's NOBODY oppressing ANY of us," they said "sure, okay."

*not THAT much better.

This was a really bad deal for the newly christened poor "white" people, on a multitude of levels.  And a large part of it has to do with slavery, and where the burdens fell.  Nobody is going to stay in slavery voluntarily unless you force them, particularly when it's as brutal and hopeless and universal a form of slavery as the South was busily developing.  So you have to have social controls, and those things cost time, effort, and money.  And guess where the burden of that fell?  Not on the elite who were the ones PROFITING from slavery, no.  On the poor whites.  For example!  In order to prevent slaves from running away in the night you have to have white people out patrolling every night.  This was not, by and large, a paid position.  Every able-bodied white man in the area would take a turn at it, usually one night a month on a rotation.  And you'd have one really rich guy with LOTS of slaves, he's getting BY FAR the most value out of this, right?  But he only spends one night a month doing this, and the next day he can sleep in because he doesn't have to work, his slaves do it for him.  Then there would be a couple of what middle class whites the south had, and they'd have a couple of slaves, so they were at least benefiting some, and they could MAYBE sleep in but unlike the rich dude, they would have to work the next day.  But most of the guys on the slave patrol were poor guys who owned no slaves and would NEVER be able to afford any.  They are getting no benefit out of this whatsoever.  And they don't get to sleep in the next morning, they are going to have to work sun up to sun down just to feed themselves and their family.  They are, as a class, putting in like 95% of the work needed to maintain the slave system, and getting 0% of the wealth out of it.  It's a great scam.

So while all that's going on, the North has slaves but not huge numbers of them, and instead of worrying about "how to control the lower class" they're worrying about "how to expand the middle class."  And this has huge economic consequences.  Not only are their economies totally different, the North's economy is A LOT healthier.  Like, orders of magnitude healthier.  But in the Colonial period, at least, this is not apparent on the surface.  Nobody's really figured out much about GDPs and the like, and so the South looks awesome on the surface.  They produce expensive raw materials like indigo, rice, and cotton, which they then sell at a hefty profit.  They've got a lot of REALLY RICH FAMILIES.  They glitter.  The northerners ... just don't glitter.  Even if it weren't against their whole Puritan schtick, at this point they don't have anybody as fabulously wealthy as the southern elite.  But they also don't have the sort of desperate poverty that is normal in the south for much of the population, and what they DO have is a vibrant and growing middle class.  So even if they aren't as impressive on the surface, their society as a whole has a larger GDP than the south, and unlike the South's economy which is largely stagnant, the North is growing great guns.

Before we go any further, I don't want you to take away from this that the Northerners were saints or anything.  They were complicit in their Southern bretheren's work, as "Molasses to Rum to Slaves" from 1776 points out so powerfully.