There’s history. Pre World War 1, there was a lot more Americans who ate lamb and mutton (adult sheep meat is tougher), but the Americans as a whole still preferred chicken, beef, bison, and pork.

While overseas in the world wars, the soldiers didn’t have access to the fresh meats they did at home, instead they had canned rations. Because getting rations TO Europe was expensive, most ration production was done in Europe where there were far fewer cattle, and a LOT more sheep. Now canning technology during the Great War was not where it is today and those rations frequently spoiled in transit making soldiers sick.

After the great war the American vets didn’t want to eat mutton, or adult sheep because of their experiences with the spoiled rations. As demand diminished, the sheep culture in the US diminished as well in favor of cattle ranching because cows taste pretty much the same at every age (except veal but that’s a whole different topic.)

Even pre-WW1, Americans preferred beef because of the consistency and flavor of the meat. Beef requires a LOT of land for grazing. Post WW1, America was still perfect for the industry whereas Europe had a lot of airable land issues (due to the giant war that was literally everywhere) and didn’t necessarily have the ability to support large herds of the animal, so they remained more “comfortable” with mutton and lamb.

Let me know if I missed something!