I discovered there's a reason for that. Apparently, it's because you need official permission from Nintendo to play games, but the older ones are all allowed by them.
Discussion
I used to think it was just because they preferred it lol, but that explains why so many Japanese VTubers play older games.
That makes a lot of sense to why they play older games mostly, well Nintendo bureaucrats are not smart are they ? Because this old games outshines many newer ones without any help and then they go and place a barrier to stream the new games and lose the chance of natural promoting them lol