Hmm.. just thinkg, instead of limit 3 invoker AND invocation AND reserve fusion AND palace of elemental lords AND why they didnt just limit 1 purgatrio and magelica +ban cocytus? Too many limitations just for 1 card.
I think that banning cocytus instead of doing anything to purgatorio was them showing a last scrap of mercy for this deck if you still want to play it (and I don't). Invocation to 3 was already the logical move. But they also wanted to hit a fusion to give a proper hit. Purgatorio is still the main offense and magellanica was already 2, so cocytus banned made the most sense.