Enterprise sank because Berman fumbled the first two seasons really really badly.
For the first two years they dropped Star Trek from the title. They wanted to prove they were doing something different and new and wanted to appeal to all of the viewers who thought that Star Trek was for dorks. They lost brand recognition, they lost casual viewers, and they alienated existing fans.
For the first two years, as I mentioned, they wanted to do something different and new. They didn't. It was the same show, only with more fan service and even less internal consistency. They wanted to make the ship all low-tech and keep technology from becoming a deus ex machina, so in order to accomplish this goal Berman decided to give his new crew more primitive things like "teleportation pads" and "phase pistols" and "photonic torpedoes" and "energized hull plating" and "particle force fields" that are all completely different from the silly and dorky technobabble science crap that made the other series bad!
For the first two years, the series revolved around a "temporal cold war" that nobody cared about. None of the writers could figure out how to give their primary antagonists things like objectives and motivations and even deliberately left the actual identity of the opponent unclear in some attempt to mask the massive plot holes with having a time shifting enemy not just sending more agents back in time an extra 5 minutes earlier with an inexplicably huge amount of viewer apathy.
I liked season 3, but they took it so far in the opposite direction that the only thing identifying it as a Star Trek series is the fact that they re-adopted the Star Trek title.
Season 4 was good, but it was also the reason the series finally failed because they switched over to a 2-3 episode arc model and many of the episodes were pre-empted or aired out of order.
All-in-all the series wasn't too bad - especially once Berman ceded creative control - but the damage was done really early on.
Rick Berman, by the way, was responsible for TNG's downswing (especially those dreadful Worf-centric episodes), personally hated DS9 (some say it's because he didn't create it, others say it's because the series was actually good and entertaining TV burns him like holy water does), thought Voyager was the pinnacle of television, and has more recently resigned from his creative control of Star Trek which is really executive speak for being fired. He and Braga almost killed the series and I'm really glad they're gone, but the new Star Trek movie is going to have to be a damn good reboot to make up for what they've done.
