Because it didn't focus on graphics over content. The content is to do with telling a deep and involving story that you get to interact with. In that it suceeds (according to reviews at least). Obviously, there are more people that want this kind of experience than most of us thought.
Furthermore, there hasn't been a quality AAA adventure title for quite some time whereas the FPS market is heavily populated with big budget, well known franchises and titles. Even though there aren't many FPS games on Wii, it does have its share of Medal of Honor and CoD games which are far better known. The Conduit was also classed as fairly average by the reviewers, whereas Heavy Rain is critically aclaimed gaining very good scores. It's also quite original and their really isn't much on the market that you can directly compare it to. Innovation and risk can sometimes reap rewards.