Pretty much everyone on the planet loves Kirby. Even if you hated to fight him in say, Super Smash Bros. 64 you still loved him deep down. The little puff ball is impossible to hate: its creator HAL Laboratory made him that way.
Well it turns out he's also extra special, so much so that he trumps the rest of the Smash crew from the annals of Nintendo history. According to Super Smash Bros. Ultimate's director Masahiro Sakurai, Kirby survived the apocalypse in the last trailer (that you can watch again below) because he wasn't "normal."
Speaking to Famitsu (as translated by Source Gaming), Sakurai says they needed a "convincing reason" why Galeem, the new big bad, could be bested by one person, who would lead the charge and eventual revolution against it. He notes that even characters with limited magic/powers wouldn't be enough to best Galeem, so Kirby was chosen because of his Warp Star. The only other "contenders" were Bayonetta and Palutena (citing their power from a lore standpoint), but their enemies were either too strong to overcome or it wouldn't make sense from a story standpoint for them to run away.
It's a cute reasoning (and a bit of a pat on the back for those two), but Kirby is also much more family friendly than Bayonetta and more marketable than Palutena. Still, hearing Sakurai explain something as minute as a trailer/story detail is why he's so great.