Much like other tragedies, people refuse to believe it could have happened. There's no way someone could just go crazy and kill a bunch of kids. There has to be more. Some nefarious evil force with an evil plan is actually responsible rather than something so senseless and pointless.
Although I do find the bulldozing of the school rather silly and wasteful. If we bulldozed every building that something bad happened in, not only would there not be a single building older than a decade or two standing, but we'd have no physical history to look at and teach future generations.
They didn't bulldoze Pearl Harbor and start over. Instead, they made it a part of history.