Super Mario Bros and The Legend of Zelda got sequels that were completely different games from the originals (and were a lot worse than the originals).
I don't know why Metroid outsold Metroid 2... I hate Metroid in B&W and maybe the Game Boy audience wasn't familiarized with the first game, that might have something to do.
Pokémon Gold & Silver came out for the GBC, that sold much less than the original Game Boy.
I don't really know the Kirby franchise so I won't comment on that.
On the other hand super Mario bros 3 did outstanding! Also TMNT 2 for Nintendo comes to mind w which was one of the best multiplayer games at the time