If Trek was so popular in 1971 that they made the animated series...
Then why didn't they commission a new TV series?
Surely a huge moneyspinner?
By the time they made the animated series, Star Trek was gaining a whole new audience. It was showing continuously in syndication, and fan conventions were starting to be held in big cities. As an existing "intellectual property" that was starting to take off in ways no one had ever seen before, the animated series was just an easy way to introduce young children to the show.
At that time, TV shows that had been cancelled were never heard from again. There were no sequels, prequels, reboots, or updates, even for very successful shows. Star Trek had the taint of failure, of being "old hat", and was expensive to produce. It was far cheaper to make an animated show than to make a new live action series.
Also, Gene Roddenberry still owned the rights to Star Trek and its characters, and he was far too busy with new ideas/projects (most of which were one-and-done failures) to think about a new live action show at that time. With the animated show, all he had to do was "sign off" and collect residual checks. I don't think he had ANYTHING to do with creating/producing the animated series - it was all done by the Filmation studio.