It was my understanding that the old process had difficulty meeting new EPA standards in the early 80's and at the same time much of the equipment used was wearing out and the old craftsmen were retiring.
As several have noted, it was actually a combination of the things mentioned above. EPA, equipment, change in manufacturing processes due to retirement of some of the craftsmen making the pistols. Remember, 30 years from now, the ones being manufactured today will be treasured classics as they will have withstood the test of time. Semper Fi!
Just go to a car show or car cruise. Those late 70s cars with no horsepower and catalytic converters are now "collectible" or even antiques. Who ever thought an imitation wood grain covered K car would be in a car show.
The EPA shut down most blueing processes in the USA due to environmental concerns. Blueing Salts are highly corrosive and air borne fumes can be deadly. The cost was just to great to keep the blueing process going. Most gun are now made out of Stainless Steel and a black satin finish made to make them look satin blue ..