I know from a Skywest pilot friend it has to do with Cat III equipment. The planes are equipped with the necessary equipment and the pilots certified, but Alaska doesn't want to pay for the capability, so its not part of the contract. Obviously if its necessary in an emergency or other life risking scenario they will land. So high fog/weather they won't fly, resulting in a cancellation/delay. The planes arent any less safe, its litterally just a contract thing.