It goes back further than that. Radio followed the same pattern in the 30's and 40's. On old time radio (and early TV) shows were owned and produced by the sponsors, not the network. Sponsors (though their ad agencies) bought a time slot and put their own show into. The ad agency produced the show, and rented facilities from the network. Shows were done live for 39 weeks and then took a 13 week break. No re-runs. The agency produced a summer replacement series for the same sponsor.
The pattern came about because agencies discovered or believed listening went down during the summer. Also a greater percentage of the population lived in rural areas before World War II. Sky wave propogation was important to reach that population on the big 50kw clear channel network affiliates. During the summer months, those stations did not have the same reach during the early evening hours. It made sense for advertisers to follow the school year calendar and go with cheaper replacement shows during the summer. And the summer shows were a good way for the agencies to test and break in new talent and program concepts (two later hosts of the Tonight Show, Jack Paar and Johnny Carson, got their start as summer replacements for Jack Benny on "The Lucky Strike Program").
Cars were never the biggest sponsors of network radio and early TV. Number one was cigarettes, followed by consumer package goods (food products, household products, health and beauty products).
>
> BTW, do you know why new TV shows used to be brought out in
> the fall? It coincides with the automakers used to bring out
> the new model year cars primarily in the fall...and in the
> 50s/60s they were the primary sponsors of TV shows
> (Chevrolet sponsored Bonanza, Bewitched, Route 66 among
> other shows, Chrysler sponsored Bob Hope's programs, and
> even Studebaker sponsored Mr. Ed!)
>
