Just as TOS was a series that deserved a better budget TAS needed animation that wouldn’t be primetime for another twenty years.
Honestly, Filmation's work just 3-5 years later was markedly better, since it improved its techniques and added methods like rotoscoped stunt and movement sequences, backlit moire effects, and more. Look at
Flash Gordon or
The Lone Ranger and it's a good deal better than 1973 shows like
Star Trek and
Lassie's Rescue Rangers, though still more limited than what we're used to today.
I've said before how much I wish Filmation had done a second TAS in 1980 or so as a follow-up to ST:TMP. That was when the quality of their animation and music was at its peak, and when Filmation started to recruit impressive talents like Michael Reaves, J. Michael Straczynski, Diane Duane, Paul Dini, and Bruce Timm. And Filmation did occasionally do sequels to earlier series, including
Batman, Fat Albert, and
The New Adventures of Gilligan, so it would've been possible. Plus they could've done a lot with the alien crewmembers only glimpsed in TMP.