broke the Cardinal Rule of sequels: go bigger than the previous installment. And if you can't go bigger, go bigger in another direction.
When it's "just another adventure" people start wondering what's the point of going to see this story in theater when there are stories just like it on TV? It's what also did in TFF.
The only difference with the TOS films was that TFF didn't flop quite as bad as NEM, the TOS movies (excluding TMP) cost less, TNG was keeping interest in Star Trek not only alive but increasing, and there was a 25th Anniversary to make the audience nostalgic. The TOS movies also had the Genesis Trilogy to fall back on, so TFF looked more like a freak accident that people could forget. The TNG movies didn't have that same batting average to back them up after INS and
NEM performed the way they did.