I can’t think of many things that make it better to be on Mars with no Earth support (even if your colony’s pretty well-developed and capable) than a survivor on Earth after a disaster other than something like a dwarf-planet-sized impactor. Climate change, nuclear war, and a dinosaur-killer asteroid combined wouldn’t do it. So-called “snowball Earth” is preferable. Earth with everywhere but the poles too hot and arid for crops is preferable. Mars is very hostile. Especially if you allow for your Earth-survivors having equipment like the Martian colonists do (far cheaper to provide, redundantly and distributed and hardened and with regular up-keep and replenishment, on Earth than on Mars). I can come up with some sci-fi scenarios, too, but as far as stuff that suddenly makes Mars better to be on than Earth and could happen today, it’s basically just super-huge impactors. Which isn’t nothing, but it’s a pretty small slice of the apocalypse pie.