From a business point of view it is smart. They know the games will sell probably anyways, and this way they will have people pay for a yearly fee to store their old mons on the Home instalment, while people can only use a selection and they won't unsubscribe because they will lose their precious other mon, which they can't recover otherwise. I wouldn't be suprised if between now and a few years, the competability between Home and the 3DS also will end, so you basically will have either have to rely on what you already transfered over to Home or are forced to buy new games to be able to have acces to certain mon.
It surely has nothing to do with data or development, since SM/USUM already had acces to everything we had so far, and for sure the Pokémon Go app also will keep being updated to include everything eventually. I think they never used the full capacity of the consoles or the cartridges ever so far, so that can't be it, correct me if I am wrong though. Also they basically use updated sprites/models from the past if we judge the trailers so far, so they easily had most already ready or close to being ready.
Also I don't 'trust' their way of thinking what 'fits the region', I mean we are talking about a company who did a Hawaii based region, that excluded the Hula Dancer inspired Belossom, and trolled us with it being available as some kind of easteregg, but not being able to be caught.