There are the usual reasons already mentioned above.
I'll add a new one--
Before it got out of the gate, ENT faced a LOT of preconceptions from fans.
Being a prequel series, fans already had their own ideas about what a pre-TOS series should be like. This goes for the design of the ship, to the history of the future, to the technology depicted, to the types of characters, to the look of the sets, everything.
I think that played a major factor too. The execs forcing the Akiraprise design and the same old tech into stories with a changed name (at best) certainly didn't help the show's perception issues. But I do think that Enterprise could have won over more people if the show's writing had been stronger.