In fact, "The Undiscovered Country" is still remembered as one of the best Star Trek movies and was the highest grossing film of the series UNTIL Star Trek XI.
The Spinoff Series do have their fans and followers, AND they have a lot of media and merchandise involved, but pound for pound they haven't performed nearly as well as the star trek adapted from the original series and original cast. CBS has probably discovered this in their own research, so the new spinoffs and films are being more closely connected to the original series than the later spinoffs.
By 1991 TOS was waning, TNG had eclipsed it at that point (later cinematic outings not withstanding) and STV had been a disappointment. TUC was a last hurrah, helped by the fact that TNG being what it was meant it could be made (sets reused, public eye etc)
By 1991 Trek was more than TOS. There’s also no way those movies would have continued past about 1989 without the TNG propping it up. And I love the movies, more than TOS, including STV. But they would have been gone by 92 at a push. TOS was not appealing to the teens at that point.