Hitting through subs is literally it. Moonblast has only 5 more power than Hyper Voice, which equates to only tiny amounts of extra damage. On the other hand, you lose all your damage if you hit a sub with Moonblast, so there's no reason at all to actually use Moonblast over Hyper Voice. This is especially true for Sylveon, as Moonblast's additional 30% chance to lower Special Attack is unnecessary for the most part. Sylveon's Special Defence is already huge, so there's next to no benefit for having what amounts to the same move but can't hit subs but sometimes makes the opponent a little weaker until they switch out anyway.
Tl;dr Moonblast is worse on Sylveon because lowering Special Attack doesn't matter and hitting through subs is more important.