Well, I wouldn't say it's terrible. Some people like it. That's just your opinion.
Enterprise suffered the same fate that many prequels do in that it did not introduce new elements into the star trek universe. In short Enterprise just close some continuity loop but really didn't present us with a unique and dynamic story in the same way the DS9 or Voyager did.
In addition, like prequels of long-running series, Enterprise messed with the perconcieved notions that many fans had about the serieses.
ST:ENT didn't suck. The reason cited for why it was cancelled, was, of course, declining audiences, and hence declining advertising revenues. One of the contributing factors was that UPN stations were only just starting to be carried on DirecTV during the fourth season of ST:ENT. ProfessorTrek 04:59, July 1, 2011 (UTC)
It wasn't my favorite series, but it wasn't all that bad. A lot of people simply didn't like the very premise of the show; going 227 years back to the first Enterprise was (for some) a big step in the wrong direction for what is supposed to be a forward-looking franchise.