In any worldwide disaster on earth, there are going to be a lot of people competing to survive. Building large fragile structures to, e.g., keep the now poisonous atmosphere out because people outside of those structures will have a tendency to destroy them. On mars, you don't have that, you have a relatively stable society where everyone is in one boat together.

If your on mars you also know exactly what you're going to face. The temperature, lack of significant atmosphere, and dust storms. That's pretty much it. You can prepare. On earth you don't know what's going to happen next. Significant die off of oxygen generating plants creating an unbreathable atmosphere? Lack of water? Too much water (flooding)? Hurricane? Nuclear war (or super volcano/asteroid, but those are less likely) throwing up dust and blocking out sun? Unmaintained dam breaking upstream? Unmaintained pipeline poisoning water supply? Earthquake? You can prepare for any one event, even in a post apocalyptic world, better on earth than on mars. But you can't necessarily prepare for them all as well. And even if you could, you won't be motivated to because you can cut corners.