TNG back in the mid 80's is what sparked the current Trek phase for most people - it was new, different and exciting. Now im not suggesting that DS9 and Voyager werent at times 'new, different and exciting' BUT they were basically a re-hash of the TNG show... with all things people (mass audience not just hardcore fans) get bored over time, and im afraid that this is probably what happened with 'Enterprise' and the current Trek phase. It just got 'old.
Ive said this in the past and I stand by it, to re-launch trek they are going to have to do something different -as different as TNG was from TOS!
It happens to most shows- the good & and the bad ones.