Because given his appearances in the other Books, there seem to be two possibilities: either Metzen’s ability to do the voice is not what it used to be, or they have somebody else voicing Thrall at least in this game lately.
I wouldn’t mind this so much if the new voice sounded similar to the old one, but Thrall sounds like a tired old man a lot of the time instead of the young and vital orc he’s supposed to be. (Reminder that canonically, Thrall is actually a bit younger than Garrosh, so for him to sound like Drek’Thar or a similar character is not only jarring, but it contradicts the lore.)
Anyway, it’s one thing to sit through the occasional appearance in somebody else’s story, but I’m not sure I can handle listening to this Thrall narrating through the course of 8 fights without losing my grip on sanity.