The airline industry was severely affected by 9/11 due being the weapon used in the attacks; then, the U.S. government prohibited the commercial fleet for 3 days, causing massive industry losses.
Congress also executed changes to improve security, so the "TSA" (Transportation Security Administration) and "Homeland Security Dpmt." were created.
Thus, extensive security precautions on commercial airlines were established, and they came at a cost, like the increment in the time cost of travel, and the decline of potential travelers.
So, the answer is:
"Costs increased because airlines had to improve security".
