I think the fact is that TNG was simply harder to translate into something mass market audiences wanted to pay to see on the big screen. Sometimes something that is magical on TV simply doesn't work on the big screen or vice-versa.
There is not much sense of nostalgia when a show moves to feature films only a few weeks after a highly successful seven-season TV run. "The X-Files" had a similar problem, which had a feature film between
TV seasons. (Ditto the 60s "Batman".)
Here in Australia, where colour TV didn't come in until late 1975, we had a few screened in b/w
prime time TV soap operas ("Bellbird", "Number 96", "The Box") release a feature film version in the 70s that drove audiences wild, not because they were great movies, or even good examples of what made the shows popular on TV, but because fans could see their heroes in full colour for the first time.
TNG on film needed an extra hook.
TOS movies came with ten years
of anticipation of seeing our heroes together again. Gene Roddenberry even suggested to Paramount that TOS be pulled from TV syndication for the months leading up to ST:TMP's premiere, to make audiences even hungrier. And it worked. Even though many ST fans were disappointed by TMP, they still went back to see it over and over and over.