I was just thinking most of our holiday foods are traditionally southern, or soul food. Sweet potatoes, Mac n cheese, mashed potatoes, and green bean casserole.

I think there's a couple flaws in this premise. First off, are all the foods you listed "traditional" holiday foods in the United States? I would argue that the foods most associated with the holidays continue to be turkey, stuffing, mashed potatoes, cranberry sauce, pumpkin pie, and to some extent, apple pie as well ("as American as apple pie"). And all these dishes were already associated with Thanksgiving in the earliest instances of preserved Thanksgiving menus. One is the "Thanksgiving Dinner" section in the 1845 cookbook The New England Economical Housekeeper, and Family Receipt Book by Esther Allen Howland, which lists "Roast turkey, stuffed" along with mashed potatoes, cranberry sauce, pumpkin pie, and apple pie, as Thanksgiving dinner dishes.

Even earlier, an article in the New-York Commercial Advertiser published in November 1817, and then reprinted in the December 19, 1817, edition of Poulson's American Daily Advertiser in Philadelphia, followed by a reprint in the December 30, 1817, edition of the Hartford, Connecticut, Times gave an estimate of all the ingredients sold statewide in Connecticut that year for the Thanksgiving festivities:

Bill of Fare of Thanksgiving Dinner in Connecticut, Nov. 1817. Geese 50,000, Turkeys 5,500, Chickens 65,000, Ducks 2,000, Beef and Pork, 25,000 lbs, Potatoes 12,000 bu, Turnips 14,000, Beets 4,000, Onions 5,000, Cheese 10,000 lbs, Apple-Sauce 12,000 gls, Cranberry do. 1,000, Desert. Pump. Pies 520,000, Apple Pies 100,000, Other pies & Puddings 52,000, Wine, gls. 150, Brandy, gls, 150, Gin, gls 120, Rum, gls, 1,000, Cider, Bran., & Whiskey, 6000. Which would take 650 hhds, of strained pumpkin; 81 do. molasses; 4060 lbs. ginger; 7000 lbs. allspice, 86,666 lbs. flour; 43,333 lbs of butter or lard; 325 hhds. of milk of 100 gals each; 1000 nutmegs; 50 lbs. cinnamon; 43,5000 dozen eggs--all which would weigh about 504 tons, and would cost about $114,000.

While some of these dishes are no longer associated with the holiday, turkeys, potatoes, cranberries, pumpkin pies, apple pies, and some of the drinks (cider, brandy, whiskey, rum) are all fairly strongly associated with Thanksgiving, Christmas, and the holiday season. The raw ingredients listed suggest they would be used for making pumpkin pies and/or gingerbread and gingerbread cookies.

Thanksgiving didn't become an annual national holiday until after the Civil War. Before that, it was irregularly held in all states, except in New England, where it was annually held in preference to Christmas as the big familial feast of the year. (See: The Battle for Christmas: A Social and Cultural History of Our Most Cherished Holiday by Stephen Nissenbaum, or Christmas: A History by Penne L. Restad for some discussion of this.) Up until that time, and arguably throughout the 19th century, pumpkin pie was very strongly associated with New England. All these other dishes (roast turkey, stuffing, mashed potatoes, apple pie) were dishes known throughout the United States, both North and South. According to the Oxford English Dictionary, the earliest instance of "mashed potatoes" appearing in print is in the 1747 cookbook The Art of Cookery, Made Plain and Easy published not in America, but in London. There are enough mentions of potatoes throughout the colonies in the 17th and 18th centuries that it's presumed that it was a common dish everywhere in the colonial U.S., and was not viewed as a regional dish.

Sweet potatoes, on the other hand, have historically been more associated with the South than with the United States as a whole. However, contrary to your argument, there are still articles being published in the present day (1, 2) that say it is still a regional preference, while Northerners still prefer pumpkin pie. In any case, sweet potato pie appears to be a regional substitute for the historically New England-based pumpkin pie, but both dishes are known throughout the U.S., and you may find one or the other or both in households every holiday season. Likewise, in those early 19th century menus mentioned above, you will see squash listed among the dishes in New England, and as far as mashed sweet potatoes may be associated with the holiday, it's historically been a substitute for mashed squash dishes in the North. But again, going back to the original point, I would argue that mashed sweet potatoes are far less associated with Thanksgiving or the winter holidays than are mashed white potatoes.

Beyond those, while in some families, macaroni and cheese and green bean casserole may be served annually, I would argue that they do not have a particularly strong connection to holiday cuisine. Green beans in general might, and early Thanksgiving menus do talk about seasonal vegetables in general (beans are a dish that can be kept in storage throughout the winter months, often dried and essentially re-hydrated), though I would argue that green beans fall behind many other dishes in their association with holiday cuisine. And macaroni and cheese isn't strongly associated with the holidays at all.

The second flaw in the premise I see is: are all the dishes you listed actually Southern cuisine? While certainly they have become associated with barbecue to varying extents in recent years, the question really should be reversed: how did some of these regional, and perhaps in some cases foreign, dishes come to be associated with Southern "soul food"? That's not a question I am going to answer here, but just going through the dishes you listed one by one:

• As mentioned earlier, the phrase "mashed potatoes" first appeared in a 1747 cookbook published in London written by an English writer. Potatoes were a common crop throughout the United States, and were not strongly associated with a particular region. As such, mashed potatoes were not associated with the South in particular. The dish appeared regularly in 19th century cookbooks printed in New England and New York.

• Macaroni and cheese has not historically been associated with the South. While Thomas Jefferson is sometimes credited with popularizing the dish, according to the Monticello Foundation, he was likely not the first American to introduce the dish to the U.S., and even in his notes on the dish, he mentions that it is Italian. While the dish did appear in English cookbooks irregularly in the 19th century, as I wrote in a previous post, the dish did continue to have association with Italian cuisine, with the words spaghetti and macaroni being used as synonyms for what we would refer to more generally as "pasta". Beyond that, the dish was really popularized in the early 20th century when it became a packaged dry food dish, particularly marketed by Chicago-based Kraft Foods. The dish's association with Southern cuisine appears to be recent.

• Green bean casserole is another that appears to only have more recent association with Southern "soul food". I have written briefly in this sub before about the marketing of casseroles throughout the United States, which appeared to be a simplified substitute for savory pies. And green bean casserole in particular does not appear to have Southern roots, but mentions in literature indicate it was adopted throughout the U.S. as casseroles were in general (examples 1, 2). There were also similar dishes adopted from outside the United States. While it's anecdotal, I know in my own family, the green bean "casserole" dish we prepare is one handed down from Hungarian ancestors, a modified, thicker version of that country's dish known as zöldbab főzelék. But the point is that it's not a particular dish of Southern "soul food", since it's not hard to find Eastern European casserole-like dishes involving green beans (1, 2, 3), an immigrant group more strongly associated with Northern industrial cities (Chicago, Milwaukee, Pittsburgh, Cleveland, Detroit, Brooklyn) than with the South.

And to this point, while cole slaw is not particularly associated with the holidays, it provides a good analog. It may be associated fairly strongly with "soul food" and Southern cooking today, and it is offered at fast food restaurants such as KFC and "Louisiana style" Popeye's, it's actually a Dutch dish introduced and popularized in New York state. The phrase itself is Dutch for "cabbage salad". So, as far as any of the dishes listed go, it seems more probable they were dishes introduced by immigrants first, and came to be associated with Thanksgiving by these immigrants, only for these dishes to be associated with Southern "soul food" later on. So the question should be reversed: how did Northern and/or immigrant dishes become associated with Southern "soul food"?

(cont'd...)