My guess is that considering the number of increasing Pokemon which almost adds to 1000, they felt that they couldn't keep up with that,
they can, pokemon is the franchise that generates the most money in the world, they can hire new people to help then, but they dont want to, gamefreaks has like 140 employess, half of them are working in town,
compare that to the thousands of employess that other companies have and get less money with their games compared to pokemon
Sure it's easy for people to say "That must've been an easy decision for them" (I'm sure it was a hard choice for them to make) But there's also the matter of data storage to think about, not to mention the obvious stress that these game developers go through.
¿data storage? if the 3ds games could have all the pokemon until then, then switch can with even more, also,about data storage , ¿you know there are like +100 lily models in pokemon sun/moon, all with their own animations for every single scene where she is, because gamefreak didnt know how to use the same model for all the cutscenes she is in?, the same happen with most characters, but its more severe with lily, so...if there are any problem because data storage ,im sure its not because "they simply cannot have all of it"
also about the stress, if they hired more people, then it would mean not only less work for everione, but you could make more for the game with less time,
also , if gamefreak wast liyng about the reason why every pokemon isnt in (because lets be real, all they have said are lies, we have seen animations reused not only for pokemon, but for trainers) and four months wont be enougth time to change anything
Heck I remember reading stuff about Sakurai's stress when it came to developing the Super Smash Bros. games. Regardless, everyone is entitled to their own opinion, we all have different tastes speaking in terms of video games. Sure it's also easy to say that they are lazy but people need to see things from GameFreak's Point of view. Honestly, I don't care if people hate sword and shield, they should at least learn to be respectful about it.
the difference is that sakurai tries to do his best, , he never said outrigth lies (at most, he changed his opinion about some things,most, if not all for the best)and the fans are grateful for the quality of his games (excluding toxic people, but you cannot completely avoid these people in any fandom)
i think if a lot of not toxic people who genuinely showed interest in sword and shield are suddenly angry, it not without reason
but anyway, pokemon will sell no matter what they do, so they can do anything they want