I recall a newspaper article contemporary with the cancellation. It said NBC simply found that kids were willing to watch the same few ST cartoon eps over and over (and I sure was at age 11-12), so there was no need to spend money on new ones.
Yep. The standard operating system for animated series in the 60s, 70s and early 80s, was to seed the usually-shorter second season with repeat episodes. To the ongoing bemusement of Saturday morning programmers, viewer numbers would not waiver whether the episode was a brand new episode or a repeat.
Contractually, it was also very common for an animated series to restart under a new name
every few years, rather that have all the Season Three clauses kick in, such as big jumps in actor salaries. (eg. "The New Adventures of Gilligan" becoming "Gilligan's Planet", etc.) The best example of this strategy is the 80s and 90s "Batman" cartoons, which often underwent series title changes and visual revamps even if the voice talent stayed the same. Sadly for the actors, it was like starting a whole new series, and their salaries would be pegged to keep the budget in check. Young viewers barely noticed.