1. Series burnout, because Star Trek had been on the air constantly for about 20 years at that point.
2. Genre saturation; TNG was the only sci-fi show on TV at that point. When Enterprise came out, there was a whole channel for it.
3. Storyline glut; TOS/TNG had piled on so much history into what had happened that Enterprise became a historical. And quite frankly, few people are into seeing a historical, and Star Trek fans are notoriously "wankish" about details. And those who aren't Star Trek fans don't care and watch it.
4. Tech issues; when TOS came out, there was all sorts of future tech, which was cool. Enterprise, taking place 100 years before TOS needed worse tech. However, when Enterprise came out, our real-life tech was advanced.
5. Also, bad casting issues. Namely, Scott Bakula.
Take away the whole rest of Star Trek, the fans that WANT certain somethings from Star Trek, and the canon-obcesseds. And tell me from just a regular ol' person's point of view, what is the problem with Enterprise. Because historical-wise, the plot doesn't seem to have problems. Sure, unlikeable Vulcan, which might've caused alot of fans to capsize, but other than that, I honestly don't know what makes it so, sooooo bad, from the reputation I've heard of it. I've been watching a few, and although I've watched all of TOS, the cartoon and alot of TNG, I seriously don't incredibly yearn for tiny details and stuff like that. So far, the whole thing seems to have a good plot, I like the cast, and it's got potencial.