Well, we approach those books now with completely different mindsets and life experiences than we had when we first read them. For scifi, it's hard to recapture that spark to our imaginations that occurred when we first read the books as kids. Also, for some books, re-reading them now really brings to the forefront how our society has changed from what it was when the book was originally written, which can sometimes distract you from the story. Some stories and authors simply don't age well; there's a reason why no reprints are occurring; I think more time has to pass before they are at least deemed a curiosity or a genre tangent worthy of republishing.
Though I do think we tend to cut a lot of slack for the books that are enshrined in our memories, those that helped to formulate the person we are today.
