Because yet another nut job decided he wanted his 15 minutes of fame with a side of blood. Even one of our favorite gun banners has said of this incident "This is almost the kind of event that's impossible to prevent and almost impossible to predict." Yeah, that's University of California President Janet Napolitano talking to reporters.
So, you can't predict it and you can't prevent it. If a good guy with a gun had been handy it might have been stopped earlier, but this being California, the most gun-unfriendly state in the union, that wasn't going to happen.
They are never going to learn.