All of the main problems can be boiled down to the powers-that-be just not committing to a true "Star Trek" prequel. Whether "powers-that-be" refers to the writers or the meddling executives or both, I'm not certain. But in any case, the first two seasons were clearly more concerned with cheap pandering to what they
thought teenage American boys would want, than with the saga of "Star Trek."
I must sound like a broken record by now, but here are the specifics of how they failed those two parts of being a "Star Trek" prequel (JMHO of course):
The Prequel part:
- Ferengi and holodecks and Borg, oh my!
- Shouting Vulcans.
- Packing an entire series' worth of new alien races with inappropriate foreheads into the Alpha Quadrant, that will mysteriously be gone by Kirk's era (maybe Season 5 was set to explain this by having Archer "water" the sacred landmarks of Denobula, Planet Butterfly-Boobs, etc, unknowingly exposing these races to human diseases that wiped them out)
- Not utilizing 90% of the races already known to Starfleet by TOS
The "Star Trek" part:
- Science fiction? Diplomacy? Diversity? Emotional humility? What kind of sissy nerds want that? This show is about sex, the South, and good old American machismo cranked up to 11 (thousand)
- Hey, we have diversity! We let a Black guy and an Asian girl onto the bridge (with the lowest ranks) and even allow them to speak occasionally!
- Only radical man-hating feminazis would take offense to our only prominent heroine being treated like a sexy hood ornament first and a character second, and most of the other women randomly losing their shirts and bras durring missions, or just being space-harpies.
- Don't worry ladies, we'll even it out by having the macho guy getting raped and all his comrades laugh at him for it! Equal-opportunity sexism! So your sons can feel just as violated as your daughters when your family watches "Star Trek." Don't drop the soap fellas, ha ha!
- Wait, we're in space? In the future? Should that affect the characters' basic traits and backgrounds? No, let's just take regional stereotypes from 2001 and plop them on a starship, with no alterations. Why make the engineer a space cowboy when he can just be a literal Texas stereotype. Why tie any "Star Trek" lore into the security chief's background, when he can just be a 21st century British soldier with slightly futuristic looking guns?
This is not to say that Abrams or "Discovery" are gold by any means, but Seasons 1 and 2 of "Enterprise" were just a farce.
EDIT: I realize I didn't quite answer the question. These are the reasons ENT failed to impress most Trekkies. But it also failed to bring in the new young fans it was aiming for.
Here's my speculation as to why that also failed:
- The "fan service" really wasn't. I recall one male reviewer lamenting that Hoshi's "girl next door" vibe was far sexier than T'Pol. Hoshi also being my dad's crush, I'd hazzard that giving her more screentime would've held more male nerds' attention that shoving fake tits onto the screen all the time.
- Fan service isn't enough. Seven of Nine's catsuit pulled people in just long enough to see what a fascinating character she was, and get invested in the rest of the cast and show. But someone just looking for boobs could easily find them anywhere in 2001. In that regard, ENT was competing with the entire rest of the media and the newly booming Internet, losing its niche audience in the process.
- Nothing else new: Besides the sex, everything else was also a rehash of stuff other shows had done to death and done better. "Enterprise" competed with "Andromeda," "Firefly, " "Stargate," BSG and reruns of TNG and "Voyager" by.......doing exactly what they did, but with more sex and less IQ points.
- Too much space opera competition: With BSG, "Firefly," "Stargate," "Andromeda" and "Farscape" putting their own new twists on space opera, ENT was desperately trying to keep "Star Trek" relevant, by.....putting no new twists on it at all. Except country music. Which scifi nerds are known to be all over.
"Enterprise" got "good" when the writers finally got into the 21st Century and realized that story arcs, military scifi, and more elaborate aliens was the way to go for 2000s scifi, but by then it was too late.