I'm not surprised. And I would be lying If I said I wasn't disappointed. This show from its inception was dommed from the start and several factors were involved in its inevitable downfall:
The Voyager factor. It came on the heels of the worst series finale of (at that point) the worst of all the Trek spinoffs. You knew some seriously bad mojo from VOY would plague ENT, whether they wanted it to or not. It didn't help that the same people involved with that show were responsible for ENT. It was like changing a coat of paint on a house that needed so much more. Which brings me to my next factor....
Berman and Braga. Let's face it. They were burnt out on Trek and saw it merely as a way to fatten their wallets. And who could blame them? There are enough "bad Trek is better than no Trek" people out there to take advantage of. They thought that the brand name alone would bring in the ratings and to hell with any effort. Why do you think it took less than a year to develop it? A good show should take much longer to write its "bible".
Identdy crisis. Say what you will about 24 and Smallville, two shows that started around the same time ENT did, they knew what they were about and sated the course. ENT? Was it primarily a 22nd century show or was it a time travel show? Was it trying to be an ensemble show or Big Three Redux? Was it about exploration or war? The founding of the Federation or the TCW?
Season Four graverobbing. What some people touted as Fanny Coto's homage to Trek's past glories was just robbing the ideas of other shows. Things like Section 31, The Mirror Universe, and the Klingons have been touched upon in a satisfactorily way to me. What would have he done any different or better that hasn't been done? It all seemed like bad fanfic to me.
Stagnation in the face of the competion. Trek's not the only game in town anymore. So why does it act like it is? Shows like Stargate, Firefly, Farscape, and now Battlestar Galactica are being written in a way that thay could challenge any non sci-fi drama with its characters, themes, and storylines. While Trek, ENT in particular, like a bloated prizefighter past his prime, continues to swing and miss and, using such antiquated devices such as the Big Three formula in the face of the ensemble formula of other shows, coasting on an ufocused arc that eventually went nowhere (did they not learn from The X-Files?), and relying on blatant titilation and needless violence (remember Airlock Archer?) to attract a demographic that really didn't care about Star Trek to begin with, continues to be a laughingstock even withing the sci-fi community.
There are so many other factors big and small, but I haven't enought time to post them. I hope that this serves as a wakeup call to TPTB and the fans that the way things have been done doesn't work anymore and nothing is guaranteed in today's climate. Trek is just another fish in a huge ocean. I don't feel sorry for the fans because I have a feeling, they knew the end was near. There just wasn't enough of them.