Spider-Phoenix
#ChespinGang
Why?
OLM has to do various weekly productions which means at times they can sometimes be understaffed or have to rely on outsourcing.
Dynamax is complex to animate since it involves a lot of effects and action must be thought differently than normal battles since we are talking about a big scale here.
Eternatus has a complex and intrincate design that demands time to really get down which just adds to the trouble pointed on first time and is literally magnified by the second.
So all in all, it's a combination of difficulties revolving how to properly adaptate that part of the game.