I think it's all really been said, but these are my thoughts.
1) the length of time between the final episode of TOS and their first movie making it a true reunion each time we saw them. Each movie was an event.
2) Character development and change, since each TOS film was pretty much seen as "the last". TNG films were always seen as an ongoing series, so whatever change they made was reset or made with an eye to that project series (Data's chip being negated and the new Enterprise being introduced). Otherwise, characters started out off model and the change was coming back to the status quo (Picard 3 out of 4 movies). Only TVH had the reset button after they tied up the loose end of "the trilogy."
3) That fact that TOS was an action adventure show and TNG was a low key sci-fi drama. TOS didn't have to change its format much to fit on the big screen (with the exception of TMP I guess). TNG became something totally contrary to what it once was.
4) TOS has three main characters and four supporting. Their films had the same recipe. TNG had seven main characters, but the movies only focused on two or three of them. Beverly might as well have been Uhura for all the screen time she got.
Whether or not someone feels TOS movies are "better" than TNG films is all opinion. The real issue is that the original series movies were truer to the concept of their original series than TNG's films were. You felt like the TOS movies were a natural progression from what came before, while TNG had to change to fit the demands of the big screen and big box office. When we saw The Wrath of Khan, we caught up with crew 15 or so years older; Kirk was depressed over his birthday, seeing his life mostly gone by and not having a command - mid-life crisis ahoy. When we watched First Contact, we saw Picard totally different from the man we knew only two years earlier; he met the Borg twice after his assimilation and didn't regress into a revenge filled maniac. In FC, he was freaking out over lines being drawn there, breaking his little ships and calling Worf a coward. He was wildly out of character for no logical reason other than to give Patrick Stewart an awkward Shatner moment (and really, Shatner was better that this sort of thing). Instead of the movies being consistent extensions of the series, they were totally separate entities.
Finally, with two other Star Trek shows on TV, the enthusiasm for paying to see yet another 23rd century based movie was severely diminished.