Wrath Classic was actually a success. Literally just look at the graph. There was a rise in subs coinciding with Wrath Classic, leading up to DF's release 2 months later.
So Wrath Classic was actually a success.
As for TBC Classic, it did keep subs afloat for a time. But then the following month we had Korthia + State-filed lawswuit + Sylvanas cinematic fiasco "i WiLl NeVeR sErVe": it was literally the darkest period in this game's history, clearly even worse than WoD.
So it's not really surprising that TBC Classic couldn't keep the same momentum as Classic 2019, Wrath Classic, and SoD.