All of shock troopers never changed from phase 2 clone armour because it was a lot better in many ways. And no one gave a dam because shock troopers were a facet of the storm troopers that were actually capable of kicking allot of ass. 

But most of those examples are from people who served in the old republic and kept their original suits. or had those suits otherwise gifted to them. As time marched on more and more recruits would have been issued standard stormtrooper armour because the older phase 2 stuff wasn't manufactured so replacement suits would be harder to come by twenty years down the road.