I think Enterprise is regarded as a failure because it ended short of it's projected 7 season "standard" run, and only fan action got it a 4th season (which ended up being widely regarded as the best).
In a larger picture, Enterprise kinda dented the illusion of the unstoppable Trek juggernaut of entertainment, the idea that anything with the Star Trek name was golden. This was a Trek show cancelled for poor ratings, something that hasn't happened in 36 years.
Now, I think that honestly, the reason Enterprise was sub-optimal was two-fold:
1. It didn't act like a Trek prequel. For the first three seasons there was little mention of notable Trek planets and events. Other than the name Enterprise and a Vulcan first officer it really didn't have a lot that made you remember this was Star Trek. When it did, they brought in things like the Borg and Ferengi, which weren't even supposed to be around at that time period, or Nausicaans that while weren't explicitly out, were definitely a TNG era creature. For it's first 3 seasons, it wasn't Star Trek: The Prequel, it was Just another Trek show that happens to be set before the rest, with a lot of Time Travel plots. One reason I think that the 4th season was so well liked by fans was that it actually remembered it was the story of how the Trek universe really began, so they threw in references to Tellarites, and Babel, and Tholians, and Gorn, and Orions.
2. UPN. UPN has made it very clear that it's trying to position itself as a broadcast version of BET, and Enterprise was a big-budget, high profile show that was way out of line with that goal, it was shuffled around like a shell game. Of course, TOS, TNG & DS9 became big hits because they were in syndication and people could see them all over the dial and all the time. Enterprise was shackled to UPN and that meant one station, playing it once (Voyager suffered the same as well), and the moved it around the dial so much that a casual observer wouldn't even know when it was going to be on. You had to stumble across it or be a dedicated fan that would keep a close eye on it. If I ddin't know better, I would think that they were trying to make Enterprise fail so they could put in more episode of some forgettable "urban" sitcom.
Personally, I wouldn't mind them just letting Star Trek rest, we've got 10 movies and 28 seasons of TV (29 if you include the animated series), that's around 700 episodes, enough that it could take you well over a month to watch it all if you were up 24/7, so really, I think the Star Trek format has been thoroughly tapped. Admittedly, somebody might come up with a really good idea, and if it was done properly (Manny Coto) and certain people (Berman and Braga) were left out then it might be good, but just making more episodes of yet another series just to make more episodes, we really don't need that.
As Orson Scott Card said in his editorian in the LA Times (
http://www.latimes.com/news/opinion/commentary/la-oe-card3may03,0,6007802.story), Star Trek became popular because it was the first really intelligent non-anthology Sci-Fi series, it was a lot more mature than the other series of the day which were clearly aimed as kiddie shows. Star Trek had a lot of bad episodes (Spock's Brain), but it also had episodes which were well acted, well written, and hold up even almost 40 years later (Balance of Terror). However, in the decades since then other popular science fiction has really taken hold, other shows have come along and beaten Star Trek at it's own game, with better continuity, better writing, more character development, and a more plausible setting and plotlines. Babylon 5, Farscape, (New) Battlestar Galactica and Lost are just some examples, while Silders did the "Strange New Worlds" idea even better (although FOX ruined the show after it's first season).
Star Trek was a great and historic TV franchise, it really brought science fiction into the mainstream and helped shepherd the common perception of sci fi away from rocketships to the moon, little green men in flying saucers with rayguns and giant monsters smashing cities. However it needs to move beyond that, it's been at the same basic level of dramatic progress for at least 20 years, and audiences have largely moved beyond that. The only thing that could have changed it, really mining it's Trek roots to fill in the gaps of the setting and appeal to the dedicated and casual fan alike, came too late.
