I realise catching them all is a pipe dream now mostly given how many there are, but not having them register in the game is just plain lazy. If they're really too lazy to write some new Pokedex entries they could just re-word existing ones - it's not like that hasn't happened before - or even cutpaste them. It's not THAT great of a task, surely. No excuse not to have National Pokedex in these games to be honest.
I think Game Freak were trying to make getting the Oval and Shiny Charms easier, and they also likely didn't want to be re-using previous Pokedex entries. In previous games when you get the National Dex, the Pokemon that weren't in the regional Dex basically recycled a Pokedex entry from the previous game.