There are practical reasons for networks to release new programming in the fall, but you might be surprised to find out one of the main reasons has nothing to do with any promotional considerations, planning, or even television itself; it was about giving the radio industry a chance to get out of New York City during the summer.
CBS and NBC originally began as radio networks, and back in the 1920s and 1930s before television, their headquarters in New York City produced most national radio programming. But as New Yorkers know, there are few things more unbearable than a New York City heat wave, and before the invention of air conditioning, beating the heat meant getting out of the city. It only made sense to shut down radio production and give people a chance to escape to Cape Cod bungalows and Catskill retreats to wait out the long hot summer.
But then television came along. Back in the late '40s and early '50s, television was just beginning to really take off. TV had clearly been on the rise before World War II, but once war broke out, the government shut down commercial broadcasts and television equipment production. Hardly anyone was working in television during the war, but the post-war economic boom of the late '40s that followed made TV sets affordable for middle-class families, and the demand for new programming was at an all-time high.
Broadcast networks knew they had a gold mine on their hands if they could satisfy that demand, but television was a new medium and it wasn't clear what people wanted to see. The easiest thing to do was to produce new shows by offering TV contracts to existing radio celebrities, and many early hits like "The Adventures of Ozzie and Harriet"and "The Red Skelton Show" were developed by giving radio stars their own television programs. Everyone making the switch from radio to TV made sure their schedules allowed them to spend the summer months elsewhere, and networks continued taking breaks after May and premiering new shows in September when everyone was back from vacation.
When television production moved to California, the tradition of starting new blocks of shows in the fall continued, and as it turns out, the system worked well for the industry—and for viewers. It still does to this day. Fewer people tend to watch TV during the summer since they are vacationing or spending more time watching summer movies. So releasing new shows in the fall ensures the networks free buzz around the annual season premieres and guarantees viewers know when to tune in to their favorite shows' new seasons.
But it's also about advertising dollars. Networks sell advertising cyclically. The upcoming TV season's schedule is revealed when the previous season ends so advertisers can buy slots months in advance—a system that began in the 1960s. Many think this way of selling ads is obsolete and should be replaced. Who knows, maybe fall premieres will become obsolete one day, as well. Many networks are already experimenting with releasing new shows in the spring and even in the summer.