According to Masahiro Sakurai, in an interview with Famitsu, the reason why Kirby (being a creation of Sakurai's) was the only one that survived Galeem's attack was because he was the only one who could plausibly escape it using his Warp Star. This also made the character accessible to starters or newcomers to the game, as a result of his speed and attacks.
Contributed by GamerBen144