Everyone Is Dead Except Kirby - Worry not, my child

Kirby's Warp Star can canonically move faster than light. It makes sense that he got away.

"Plot Armor is when a main character's life and health are safeguarded… with no logical, In-Universe explanation."
-TvTropes

The logical, In-Universe explanation is that Kirby on a warp star is the fastest fighter.
Sonic's speed is pretty exaggerated in his title of "fastest thing alive", as is plainly evident in his games. Chaos Emeralds and the Lightning Chariot weren't readily available. Kirby can just summon warp stars. Farore's Wind and Mewtwo's teleport are both limited to places you've been before. Palutena's warp has limited range. We don't even know if Shulk came up with something, as he's never shown being consumed by the lasers.

It's Kirby because Kirby is broken.