Back in the 60s and early 70s, the networks voluntarily conformed to the NAB (National Association of Broadcasters) standards for limiting the number of advertisements during prime time. There were indeed considerably fewer minutes of commercials back then.
Later, Jimmuh Carter objected 'cause he felt that such an agreement was a violation of anti-trust laws, that it artificially inflated the price of advertising, and if the networks increased the amount of available advertising time, everyone would be happy.
So, they did. And everyone's happy, right?
Every once in a while, you'll see reruns of some pre-1980 show come around, advertised as having all the deleted stuff edited back in. I think the most recent was the SciFi channel running the original uncut Star Trek. They had to run them at least 1:10 just to fit the modern standard for commercial breaks, although I think they stretched them to 1:30 and filled the balance with yammer breaks from the actors.
Another example was M*A*S*H - seems I remember some station having to block off about 45 minutes to fit the original episode, plus the current prime-time extra advertising space, plus the extra post-11 O'Clock news extra advertising space.