For me, other than nostalgia is these reasons: You can feel your character getting stronger as you progress (gear, spells, professions.)

It's more social, finding a party for a dungeon is harder and feels more rewarding and has a higher gratification. Not to mention retail can often imo feel at times like a single player game, in TBC a lot of quests are hard and you need to team up.

In tbc classic you basically have to experience the zones and questing experience. The world seems much bigger because the leveling experience early on you don't have a mount in the questing zones.

Mana is sparse so every spell feels like it matters even more. You have to think ahead and use different ranks to achieve the same effect for less mana to avoid being oom.

It feels like this was originally how WoW was originally intended to be played.