First, action-rich games present themselves much better on streams. WOW Classic is a slow-paced RPG game, which doesn’t really fit well with streams. Unless it’s a PVP or raiding stream. But, for the casual gameplay, it is sort of Big Brother - I’m watching someone’s everyday life, and commenting on it.
As of today, Classic doesn’t offer much except casual gameplay. Maybe some player-run events or PVP that deserves a stream, but to me, those casual streams aren’t that interesting.
Second, it doesn’t immerse you the way it would if you played it yourself. And, you are still getting spoiled. Spoiled of the nostalgia, because you get to see the old world, get to hear the music. You are slowly getting spoiled, but you aren’t yet in the game. When I watch a stream, I’m already getting bored of the game, even before I started playing. I’m really feeling like “you think you do, but you don’t”.
Third, Classic, as a slow-paced game, would be more suitable for small-scale streamsers, that have about 4 or 5 viewers for casual talk - rather than big streamers, who get meme bombs on chat, and carelessly ban anyone who sends a link, because “it’s just one of 500”.
Finally, streaming as a way of hyping doesn’t work - why would I watch a stream, if I can hop on a private server? That would too spoil me the same way, and hype me even more.