I think the problem there was complacency. Episodes became very cliche (Dilemma, Action scene, 'splosions, resolution). It lacked the daring of TOS, the intelligence of TNG, the intrigue of DS9. VOY and ENT did not build on what had gone before. The same thing occurred with the Movies. Every film after Star Trek IV had the same plot structure. That gets tedious and boring. The producers blamed it on Star Trek needing a rest or however they put it. Personally, I’ve always said, “That’s a cop-out.”
Star Trek is about the Human Adventure. Alien-of-the-week, mysterious virus, evil-twin, and all that is supposed to be used as a device for that discovery. Take the headlines and create an allegory for the crew to engage with commentary. In the late 90’s Star Trek avoided those topics and when it did it lost its appeal to the producers and as a trickling effect the fans.