Why is prk preferred over lasik if you want to become a commercial pilot?

Stress resistance. The long term results of lasik and prk are approximately the same. But prk gives a more solid front of the eye less resistant to pressure changes and trauma. For most civilian activities it makes no difference. The military requires pilots to have prk when needed to prevent any possiblity of problem and this may hold true in some circumstances for civilian flight.
Not sure it is. The long term safety and effectiveness of prk and lasik are very similar. Classical thinking was that prk may be better in the event of pressure changes since there is no flap. However, lasik has even been studies and approved for both nasa and military pilots.
Flaps. Prk may be preferred by some airline companies due to perception of flap complications, or postop halos or dryness but that also can occur with prk. Many commercial pilots have had lasik, and ilasik has even been approved for nasa astronauts. The commercial airline(s) of interest may have specific reasons they can share with you.
Job requirements. Long term outcomes of prk and lasik are equivalent. Requirements for commercial airline pilots allows prk but not lasik likely due to prior concerns of vision changes at elevation. The requirement is antiquated but has not been updated.