From my understanding, the vast majority of chickens in the United States before the Great Depression were raised for egg laying, while chicken meat was reserved for special events and holidays. Today, US chicken consumption outpaces that of all other meats: the average American ate 115 pounds of chicken in 2020, according to the University of Illinois. link How did this happen?
This is a reasonably easy one to answer, and the answer comes in three forms: faster-growing chickens, more and cheaper chicken feed, and richer people who live in cities.
First, the chickens themselves: by selective breeding, farmers (which query whether we should better-consider them scientists, really) got chickens that were ready for sale from 12 weeks to 6 weeks. The feed required to get them to size fell by half. The growth rate went up four times, and the amount of breast meat rose 70%, all between the 1920s and the 1980s. In 1922, a pound of chicken cost $5.50 (using today's dollars).
Second, chicken feed. Chickens are remarkably efficient at turning food into meat. Generally, the chickens of the Great Depression scratched and pecked and ate scraps and what have you; the chickens of the chicken-industrial age eat a rather more finely-tuned diet. And in the same time that chickens were getting bigger faster, corn was getting cheaper by a lot, and corn turns into chickens in a hurry. Corn yields per acre have more than doubled since the 1930s.
Third is humans moving to cities. About 1.3% of the country works on farms now. In 1920, it was about 30%. Fewer people keep chicken around for egg-laying, so consumption patterns changed.
Put it all together and you have much cheaper chicken that is more readily available, and as a consequence, a lot more people eating chicken.
There are a lot of positives in this story: more food is generally not just a good thing, it's a great thing. But there's a real down-side too. A lot of people, including me, would argue that there is such a thing as animal welfare, and that we should be at least a little concerned about it, and the plight of chickens is something that should concern us at least a little. Many others would tell you, I also think rightly, that it affects taste, too.
Sources:
MJ Zuidhof et al. 2014 Poultry Science 93 :1–13 (chicken growth rate)
RL Nielsen, Historical Corn Grain Yields in the U.S. (Apr 2020)
Michael Pollan: The Ominvore's Dilemma
Jennifer McLagan: Fat, An Appreciation of a Misunderstood Ingredient
Hugh Fearnley-Whittingstall: The River Cottage Meat Book