Sabellian hid away in Outland long after we brought down the Gronn, Magtheridon, Lady Vashj, Kael’thas, and Illidan.
Meanwhile, a whole hell of a lot of stuff was happening on Azeroth where we really could’ve used his help. Wrathion, for better or worse, was there risking himself. Fighting N’zoth could’ve easily gotten him turned and/or killed, and he knew full well how dangerous it was for him to stand against an old god.