My theory on the popularity has to do with the rewards given to the viewer by watching every episode very closely. It's like being involved in an ongoing investigation but having to do very little of the work.
Seinfeld did a little bit of this. By having so many episodes tie into another on levels that would only be appreciated by people who watched every episode, fans felt like a little club. Alias touched on this trick, but Lost went all out.
The attractive cast doesn't hurt either. Can't be the only explanation though, not many ugly people in prime time.