It would probably be more accurate to say that 3e moved Gruumsh to Chaotic Evil because it moved orcs from Lawful Evil to Chaotic Evil.
Because the fiends in the Blood War are something you won't find anywhere else in the multiverse: evil-aligned fiends who are doing nothing to hurt anyone who doesn't deserve it. Attempting to convince them that celestials are their common enemy would run a dangerous risk of succeeding.