Cheap programming is the answer. In the good old days when there were only two or three stations, the programme makers had big budgets to make quality programmes to grab the biggest share of the audience.
Because of the limited choice, programme makers could allow new series time to grow and develop - a lot of 'classic tv' was an initial failure, but over time grew to be firm favourites.
Nowadays, if something new doesn't succeed very quickly, the series is pulled.
Of course, tv went off at 11pm then - they weren't filling the schedules 24/7, so the budget had far less of a stretch.
Then came remote controls and channel hopping. It became harder to guarantee the prime time viewer - it used to be that once the tv was turned on at 7.30 or 8pm, the channel would rarely be changed until the dot on the screen that signalled 'time for bed'.