This was purely a restriction applied by higher command.
In 1971 a Royal Air Force Puma had the captains cockpit door fall off whilst returning to Odiham. There was no reason found except the suspicion that the jettison system may have been fiddled with whilst the aircraft was being looked after by army personnel beforehand. As a result of this it was decreed that the cockpit doors be locked permanently and the crew would have to crawl through the cabin to get to the cockpit. Locking the doors would have made no difference to the result if the initial suspicions were correct.
In 1978 a civil Puma had the pilot's door fall off as a result of a massive nav bag being jammed between the seat and the door which migrated forward and pushed the jettison handle and released the door.
Since then Pumas of various marks have flown millions of hours around the world with unlocked doors that have been used for what they were designed for; to enable the pilots to enter the cockpit.
Except the Royal Air Force, who still fly around with their doors locked.