Though we at GameFreak wish to see players capture and raise many Pokémon in order to complete their Pokédex in Pokémon Sun, Moon, Ultra Sun and Ultra Moon, we realize that there are a very large amount of Pokémon to seek out, and some are harder to find than others, even being completely impossible to find in Alola. For this reason, we have decided to exclude the National Pokédex once more in order to make the fantastic feat of capturing all the Pokemon that Alola has to offer feel much more rewarding, rather than be a stepping stone for an incredibly difficult feat that players with busy lives are unlikely to pursue.
You may still use the National Pokédex within the Pokémon Bank Application if you wish, though.
