I can think of at least two reasons:
-The TOS movies weren't afraid to take more risks with the main characters, which in turn made the films seem more worthwhile. For instance, Kirk tries to overcome the limitations of getting older, while Picard turns into a young-at-heart Rambo. Which character's struggle is more engaging?
-I liked that some of the TOS movies were actually *about* something. They were morality tales that covered important issues of the time - to me, that was the heart of Star Trek as a series. You had a pro-environmental message with Voyage Home and an anti-cold war message with Undiscovered. The closest we got to a real issues-based flick with TNG was Insurrection, and I'm not really sure what that flick was about - the displacement indigenous populations? Not exactly a hot topic of the time.