I'm guessing larger refrigerated trucks for transport, media blizes focusing on wholesomeness and handiness of product, the price of feed (at the time) for chickens, hormones used to speed up production, medical availability for what was projected as a healthier product line, larger stock of cartons for transport ease. I'm speculating that if this started in the 50's it was quick and cheep (ha) for meals following war. While many homes had the proverbial chicken, maybe they ditched the pot and instead kept the hen for eggs. Knowing that one or two wasn't gonna cut it on the home front, and with men coming back from war to work, women, traditionally stayed home to cook. Back then there were less packaged foods, and who in those days didn't find an egg or two hardboiled and chopped into sandwiches or salads. Hardboiled and not shelled, they were the perfect travel food for schools, and following the baby boom after returning military, I'm also guessing that schools went through a whole bunch, upgrading their refrigerators. And isn't this about the time where eggs were first successfully used in innoculations/treatments/ And when the first processed mayos and ice creams hit the markets? And packaged mixes?