>>518867843BC was always kinda overrated and the massive growth it saw back in the day was due at least in-part to WoW fully piercing into the public zeitgeist. The mass influx of new players meant lots of newbies taking their time in trying to catch up and so they got to experience all of vanilla + most of BC in one swoop essentially, further increasing the appeal, more word-of-mouth, snowball effect, etc. (This pattern is also why original WotLK's pop numbers were essentially a flat line, because you still had new people joining, but also some more long-time players were beginning to drop out)
In the case of BC Classic, the only ones really playing were those that were already prepared. They had done a full tour of Classic, probably had multiple alts ready too, and the only shit there really was for them to look forward to was just BC stuff. In the modern day, jumping into an already running online game behind lots of poopsock-players is a major deterrent for a lot of people and so BC Classic could never really get the constant stream of new people to join.
