Well, I can’t recite the history for you, but I can tell you what I remember.
The old radio shows, such as The Lone Ranger and Sgt. Preston of the Yukon, were half-hour shows. I’m guessing that the basic 30-minute format (minus commercial time) was simply carried over to TV.
Also people wanted to remember the times of their favorite shows and tune in regularly. Start times on the hour and half hour were no doubt easiest to schedule and anticipate.
I would also guess that it was easier, especially early on, to find sponsors for the shorter shows. It was pretty hard to say exactly what they were getting for their money, other than that if they advertised, they sold more than if they didn’t.
When I was a youngster, there were very few hour-long programs other than music and variety shows. Disneyland was exceptional. Most story-based programs were half an hour. I also sort of remember one or two programs that ran for 15 minutes. They usually ran back to back in half-hour slots.
It was pretty exciting when some longer scripted shows such as Bonanza started to come along.
The way I remember it, dramas migrated to a one-hour format over the mid-century decades, while comedies tended to stay at a half hour.
No doubt it was money that really decided the matter: what will sell to sponsors, what will sell to audiences, what will sell to networks.