Maybe, just maaaaaaaaybe, they are playing wow classic BC and run around in the old world still and quest/level? These people didnt choose for BC to launch and they didnt choose that alot of people move to BC. Its only natural that most people move over to the BC servers, if they are going to play BC or not.
The good thing about BC is that the old world is still intact and you can go back there anytime for whatever reason. You can also keep on leveling characters in the old world.
When it comes to raids etc - Blizzard gotta restart classic realms for people to return. That worked on illegal versions of classic, it will probably work this time to.