because Roddenberry didn't support it strongly himself towards the end....
Partly because his lawyers were fighting a lawsuit with DC Fontana and David Gerrold at the time: they were suing for co-creatorship on TNG. (And they eventually received a settlement, with a non disclosure clause.)
It makes sense to diminish the past contributions of your opposing team.
Certainly, when TAS was being made, Roddenberry supported it as strongly as live-action ST. Check out old issues of the TAS Fan Club newsletter that was put out every few months by Lincoln Enterprises.