On pretty much any table in any restaurant in the U.S. you will find a bottle of ketchup.
With pretty much any order of fries, hotdogs, chicken, or milanesa (breaded fried chicken or steak) in Argentina you will find a serving of mayonnaise.
Why the difference?
One clear and distinct answer lies in the history and invention of these two sauces. The invention of modern ketchup is credited by some to H.J. Heinz, the founder of The Heinz company. His adaptation of the Chinese “cat-sup” (still seen written this way on some company’s bottles) made its way to market in the late 1800s, originating in Pennsylvania, which explains why it penetrated the market so much in the U.S. It didn’t arrive to Europe or South America until much later.
Mayonnaise on the other hand, was invented much earlier, and indeed by the French Duke de Richelieu in, uh, France around 1756. This gap of 150 years or so with mayo and without ketchup in Europe is probably the reason for the majority of why American’s prefer and use ketchup over mayo and vice versa in Argentina, South America and Europe.
Now the hard data was really hard to come by, but I think that there is another reason for the respective preferences for ketchup vs. mayo in different regions: the palate.
Ketchup is basically sugar. With 2 of the first 3 ingredients in ketchup being high fructose corn syrup and sugar, Ketchup fits America’s need for sweet products, sweet/savory combinations. Ketchup has no fat in it and is basically entirely sugar. This produces a drastically different hormonal effect in the body when consumed (sugar high, followed by huge insulin spike, followed by lethargy).
Mayo is essentially entirely fat with the main components being egg yolk and different oils (depending on the recipe). Strangely, mayo takes on different colors in America vs. Argentina. In the U.S. mayo is starkly white. This strikes me as odd for a food that is based on the yellow egg yolk because in Argentina, it is much more yellow.
The 100% fat content of mayo satiates the appetite and gives people a full feeling. This hormonal effect of satiation is much different than the roller coaster sugar high effect of ketchup.
Not to mention that the flavor of mayo is not sweet at all. It is entirely savory, another thing that illustrates the two countries’ preferences for sugar vs. fat, sugar high vs fat satiation, ketchup vs mayonnaise.
The odd thing about the practice of serving the two condiments is that ketchup in the U.S. is omnipresent. It’s on every table in every restaurant. ANYTIME fries are served, ketchup is there, even in high end establishments. Mayo would be odd in that circumstance. I invoke here, the movie Pulp Fiction where the John Travolta character, at the beginning of the movie has just returned from Europe and tells the Samuel L. Jackson character about how they “drown [their fries] in that shit (mayo)”, and they both remark about how strange they find that custom.
In Argentina, fries are served without anything and I have always had to ask for ketchup, and since I view fries basically as a ketchup transportation system, I have been frustrated when restaurants do not have ketchup. What a nightmare.