Enterprise's first two years killed it.
The character writing wasn't the best, as evidenced by the fact that Archer was a complete bore - despite being played by Bakula, a more than capable actor. Mayweather, Hoshi, both weak too. Reed was very one-dimensional. But for Trip, T'Pol and Phlox it would have been disastrous.
Beyond that, it took them far too long to find a direction. The temporal Cold War arc was ridiculous. It was obvious that it was being made up as they went along, and the less said about its resolution, the better. Given the time period in which it was set, they had an abundance of lore material to explore, which they eventually started doing in the final year, but far, FAR too late.
Then there's the final episode.....
Voyager - strong premise, engaging pilot, and then....it fell off a cliff. The Maquis/Federation internal conflict was wrapped up in a single episode ffs. As per above, the cast was not great. Mulgrew is experienced but I don't think they wrote her particularly well. Beyond the EMH, Paris, and later 7 of 9, the rest were all very bland indeed, and Neelix just plain irritating. The introduction of the Borg was a double-edged sword. It brought in 7 of 9, but then, the Borg themselves were handled so poorly. By the time the finale aired I never wanted to see the Borg ever again. And the finale itself was so underwhelming. "Yay, after 7 years we've finally made it home"...fade to black.
Voyager and Enterprise, coupled with poor TNG movies, Insurrection and the awful Nemesis, signified that Trek was well due a rest. New people involved, hopefully new ideas fused with familiar themes and principles and a new series could be great again.