The changeover from corrosive to non corrosive was not a smooth one that took a simple change to complete. Some of the early mixtures were not stable, some deteriorated rapidly. There were problems with the non corrosive mixtures being not sensitive enough or being too sensitive, the early NC mixtures tended to be more brittle and subject to crumbling (hence the foil disc to keep the mixture out of the powder), and there were different seating requirements between the Boxer and Berdan systems. The sensitivity problems still haven’t been solved completely, witness the special milspec primers for use in some automatic weapons.
Maybe some countries weren’t able to address the problems as quickly as others and simply chose to wait. I don’t know. Just a guess.