I suspect that their standards are different. Someone who's willing to jump through the hoops to play classic WoW on a private server likely has a willingness

I suspect that their standards are different. Someone who's willing to jump through the hoops to play classic WoW on a private server likely has a willingness to deal with glitches and troubleshooting issues with running a 12-14 year old game client. Whereas Blizzard wants to provide a smooth official experience, which they're going to give to their mass market audience. They can't get away with saying "yeah, it crashes a lot, and runs terribly on New Video Card X, deal with it", because it makes them look bad. (For that matter, I suspect things like saying "that crash bug was there in 2007 as well, so it's the authentic experience" wouldn't go over fantastically...)