You know, on this note, specifically talking about Adeleine, I can't help but wonder if the timing of her appearances, particularly that in Kirby 64, simply worked out badly for her.
Kirby 64 was released in 2000 - the same year that
the Kirby anime pilot short was made, and therefore also when decisions were being made about the full-fledged production that would hit screens in Japan two years later.
Why would this affect Adeleine, though? Well, though the anime was specifically intended to be non-canon, a few small references to it were slipped into the Kirby games of the time in order to help establish and promote it. These things ranged from temporary visual overhauls (Castle Dedede, which has thankfully now been returned to its smart-looking old self
), to certain traits ("Kirby must not speak"). It's this latter one that's the problem - aside from how much Kirby speaks (Kirby was always a quiet type anyway, even before the anime, but it was accentuated far more there for numerous plot reasons), one of Sakurai's other edicts for the show was "No humans". Given the in-game nods that were going on at the time, it seems that this could be at least part of why Adeleine suddenly disappeared.