My dad worked long enough hours that he was not home to watch it at the 5PM or so slot that it was in. My mom was busy cooking dinner. It was therefore not "appointment television" in this household. It was a show that nerdy boys watched after the afternoon cartoons were over and before sitting down at the dinner table.
And that fits perfectly with the US popularity of ST in the 70s. Parents who had watched it in first-run in the 60s were happy to let their kids watch it in the 70s, whether the whole family sat down together or not. Over dinner, many families discussed the episode that had just run.
Here in Australia, we had "Gilligan's Island" and "Lost in Space" in that perpetual rerun slot, not ST. "Star Trek" didn't get a similar treatment because only 40 episodes were rated "G"
Down Under; the others were "NRC" ("Not Recommended for Children"). When rerun for "the first time in colour" from 1981, it was back into an after-8.30pm timeslot. When it then shifted to weekend afternoons, the network holding the screening rights was told that they'd have to pay for a new classification process to have the series reevaluated to account for changes in 80s sensitivities, or just stick with repeating only the 40 "G" episodes. Which they did.