For the same reason some airlines won't transport pets on hot days.

The risk of damaging the surfboard (and the resultant hassles) outweighs the loss of revenue that might come from losing the passenger's business...

Also, some airline computer systems are not set up to exclude you from a routing that might put you on a commuter flight that might be unable to transport the surfboard.

Airlines are configured to move people, not cargo. If you want to move cargo you need to talk to a courier.