>>53599767Alder hadn't appeared in the anime for like 10 years, having him suddenly return for 2-3 episodes with an entirely new voice would've just been awkward as fuck when the entire thing was completely avoidable by just... not having him appear in a speaking role.
Oak, on the other hand, was already going to appear in dozens of episodes that were in all kinds of states ranging from "plotted" to "completely finished and just waiting for the voice acting to be recorded" at the time of Ishizuka's death, they couldn't just magically erase him from all of those at a few days' notice.