The difference between all of the named series is that they have their origins in comic books or toys, which--historically--lend themselves to their natrual "cousin" in animation with greater ease than movies.
But the series I named are far from an exhaustive list. Remember The Real Ghostbusters
? That wasn't based on a comic or a toy. Nor was Men in Black: The Series
. There was also a Bill and Ted's Excellent Adventures
cartoon released between the two films; The Mummy: The Animated Series
based on the Brendan Fraser films; even animated adaptations of the R-rated RoboCop
franchises, not to mention Police Academy
. These things have never been limited solely to comic and toy adaptations. It's just happenstance that the ones currently airing are mainly based on such things, because so many feature films these days are based on them.
Further, the NuTrek films are still trying to justify their existence in a modern film world that has surpassed it in terms of interest, spectacle and basic appeal (post Potter, Rings, Hunger Games, numerous comic book adaptations, etc.), so a new cartoon based on NuTrek (or any Trek, for that matter) is a dicey concept at best. This was not the case when TAS was created, as the interest for more TOS was at a fevered pitch. That made the acceptance of an animated version easier than what ST as a franchise faces at present.
I think you're overestimating the audience interest in a new Trek series as of 1973. I grew up as a Trek fan in the '70s, and I can tell you that although ST did become something of a cult phenomenon at the time due to its success in syndication, it never had the kind of mainstream acceptance that TNG gained. Indeed, science fiction as a whole, in those pre-Star Wars
days, was much more of a niche market than it is today. So there was enough audience interest for Filmation and Roddenberry to convince NBC to do the animated show, but interest wasn't at a fever pitch among the public as a whole. I'd say the situation wasn't too much different from today, where there's still a loyal and widespread fanbase but the franchise is far from the top of the pop-culture heap. Except that SF/fantasy in general is far more accepted as a mass-media cash cow than it would've been 40 years ago.