Because he never made it that far.. He strolled up to the ticket counter and started blasting.
Now, had he bought a ticket and went to board.. THEN he would have passed through the security checkpoint..
(As it was, the security checkpoint came to him..)
Then we have learned NOTHING from the Israeli experience - shooters and bombers could walk into crowded terminal areas and kill dozens of people. I can't believe we haven't instituted remote security checks at our airports yet.