>> If they could have gotten more money by taking time away from shows in the mid-80s, then why have they only slowly been doing it over time? <<
Stations that belonged to the National Association of Broadcasters (most did) subscribed to the Television Code, a set of voluntary standards established by the organization in 1952. The Television Code prescribed self-imposed standards on matters such as programming in good taste and limiting the amount of advertising within programs. Member stations received the NAB's Seal of Good Practice. They would display this seal at station sign on/sign off and on other occasions.
The Seal of Good Practice harkens back to the era when broadcasters were considered public trustees of the airwaves, and that they had obligations to operate in the public's best interest. These concepts were eroded by Ronald Reagan's efforts to "deregulate" radio and television in the 1980s, and by subsequent acts of Congress. That's why the transition occurred over time.
In 1983, a Federal judge ruled that the Television Code violated the Sherman Anti-Trust Act and was responsible for keeping the cost of commercials artificially high. The Television Code was declared unlawful. So today, we can enjoy "Fear Factor," have almost no non-news local programming, and commercial breaks that last virtually as long as they show they appear in.