Well... given Star Trek.com is mostly frequented by fans, other fans I'd say.Underrated by whom? The poll doesn't say.
As far as the general public is concerned, it's all Star Trek. If Enterprise is remembered individually at all, it's probably for being "the one with that guy from Quantum Leap in it". They're all shows featuring aliens with funny shaped ears. Funny foreheads. A crew of 7 or 8 boldly going, with a fresh turn over in actors once a generation. Heck, ask anyone what the last movie was about and they'd say it was a prequel. They don't care about the little things, in the way a fan tends to. It was the authentic story of how Kirk and Spock met, how the Enterprise crew had always got together. Nevermind the alterations to each characters past, the ship itself and the destruction of Vulcan. It's not like they care one way or the other frankly. When maybe they should and stick around, get to know it better.