From the dawn of modern broadcast TV in the fall of 1948 until the spring of 1971, prime time was from 7:30 until 11. Then, the FCC introduced the Prime Time Access Rule in 1970. The rule took a half-hour from the networks and gave it to the local affiliates to "develop new shows designed to discuss news and politics on a local level."
If the affliates needed an example of what the feds wanted for these shows, they could tune into PBS, which launched in October 1970.
The FCC had already stuck it to the networks in 1970 by banning a major advertising segment – cigarette companies.
So within a year the government restricted the networks' ability to make money and set up state-owned competition. All this was done in the name of the public good. The feds saw the networks as growing too powerful. Of course, a year before any of these moves were made, a new company incorporated in Pennsylvania called Comcast Corporation.That would render all these fears meaningless.
But thanks to federal regulators most of the population can now watch TMZ at 7:30 while our wealthiest citizens can enjoy programs that cater to their tastes.
Thanks to Brian Cronin and his site "Entertainment Urban Legens Revealed" for the education on the Prime Time Access Rule.