Because he was a kook.
I want to congratulate all the Freepers out there. This entire story was out here on Free Republic last night on one of the story threads. Even the name of the shooter was on Free Republic last night, before he was officially identified. Now ABC is confirming this story about him buying the weapons and ammunition.
Free Republic can be a site with breaking news. Perhaps the MSM wanted to hold back this aspect of the story for some reason. And the delay in official identification suggests something too about the MSM and the officials in charge.