"Enterprise" was part of that whole millenial era vibe where prequels and remakes were dreadful, along with many sequels. And the dreadfulness was not always in being terrible. It was all too often in just being very bland and lifeless. These were the years of the Star Wars prequels, Tim Burton's Planet of the Apes, Superman Returns, etc. "Enterprise" partook in that.
The bigger question may be to figure out why that sort of quality issue existed. It may boil down to that the people at the helm didn't know what they were doing, didn't understand the vibe of the thing they were involved with, and got too caught up in CGI rather than focusing on story, landing in mediocrity. The problem with Enterprise was probably all of those things. It was also the fact that the people involved with the rebirth of Star Trek with TNG were all gone by then. Michael Pillar was no longer attached, nor Ira Behr, nor Ronald D. Moore, and so on. I can't remember all the names, but there was such a brain trust that was around for TNG which dissipated away as the franchise went on, and weren't replaced. By Enterprise, it was Berman and Braga. I think you could see the first problems with Star Trek: Voyager. Voyager wasn't bad, but it also wasn't good either. As things went on past DS9, Star Trek began to feel like Power Rangers. It was just a series of bland things that existed with a massive franchise no one was particularly enthusiastic about the current incarnations of. The shows, starting with Voyager, also suffered from uneveness; the characters did not have consistent characterization, and were constantly contradicting themselves.
The problem when it moved in prequel territory was that not only was it a problem of blandness and lousy characterization and lousy stories; it was also a problem of screwing up the canon. Enterprise should not have been as bad as it was, and it was as bad as it was for most of it's run due to incompetence. You have the problems I already mentioned. You also have the problem that it totally ignored the Original Series. The entire point of ENT was to go forward into TOS, to show how all these things came about, and to show these adventures in this rich era. Somehow, they found a way to botch that, and they also found a way to totally ignore the original series beyond some lip service they assumed meant something. The color scheme being based on TOS does not mean anything, because it was a group of people with a TNG mindset trying to look at something and pay homage to something it seems they didn't understand. Enterprise should have been about setting up how the Federation formed, and the Romulan wars, and the innovation of new technologies which would become commonplace in the later series', and showing and forshadowing all the things that would come about later, and doing the things we were told happened during the era of that show. Instead, it totally sidestepped that. In short, an episode named "Daedalus" should have been about the creation of the Daedalus class. Instead, it was about a transporter malfunction. Instead of the Romulan wars, we got the Xindi, which were never mentioned before, do not belong in the Star Trek universe, and were a total distraction. Not to mention that the entire season 3 Xindi arc was not preplanned and outlined whatsoever, and they were literally winging it as it went along.
What season 4 did was two fold: it did everything I mentioned about going into TOS, and it also cleaned up the canon mess the first three season created. How you manage to do that from the get go, I'm not particularly sure of. The way it turned out, it seems like there must have been something in that environment at that time that was the reason for all that. Certainly you could make the argument of just making it not be terrible from the word go just by having it not be terrible, and for no more reason than that.
