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Three reasons
>SS code still has one free slot in base game code for one more pokémon addition
>PBP likely getting reprinted, could probably have Shaymin included as evidenced by Gracidea lady being in Floaroma town
>Legends releasing two months after BDSP will likely have Arceus catchable in-game for the first time in series' history
There's no real need for them to include anything new and that's of course assuming they were going to bother making anything to give ILCA, when they had their own Sinnoh project on the boil they'd almost certainly wish to use any ideas in first, before bothering about the rehash, they already decided to go the faithful remake with it.