Lots of reasons. Probably because our world is larger (and our iron core may also be more dense) so generated a strong enough magnetosphere to resist the solar wind.

Sort of like ships in a gale. If your ship is big enough - you survive, but if you are in a sail boat - you're toast. A 'threshold' of magnetosphere that might be needed for a world and atmosphere to survive.

The most important take-away from today's conference for folks concerned about climate change is that the magnetosphere and radiation create desolate worlds.

Earth's magnetosphere is shifting. As it adjusts climate in various regions will adjust with it. Some areas will freeze over that were warmer and other areas that will thaw out. So arctic ice measurements that are lower are actually to be expected with the magnetosphere shift.

And more importantly, it's not just shifting right now - it's weakening. This scares me and it should scare anyone who really is concerned about climate change. The European Space Agency's SWARM array has detected drops in strength.

It's weakening rapidly in the western hemisphere.

http://www.esa.int/Our_Activities/Observing_the_Earth/Swarm/Swarm_reveals_Earth_s_changing_magnetism

Not surprisingly this is where we will experience more melting - more incoming radiation and to add personal injury to planetary change - a greater risk of skin cancers.

If the poles are flipping, some folks are guessing that it didn't cause a total temporary collapse of the magnetosphere and accompanying radiation catastrophe based on some of the sediment records, etc. But that's a big guess. The data is inconclusive and speculative.

But this won't change the current climate change rhetoric which revolves around CO2 and pollutants as the climate driver. In fact, the weakening and shifting magnetosphere will never be mentioned. In a world where moving factories from one nation to another brings billions of dollars to non-treaty nations, the real "inconvenient truth" is that pollution has almost nothing to do with any increasing temperatures, bigger storms or melting ice that someone wants to point out - it's all radiation.

Human nature is such that people like to feel in control. Recycle! Conserve! Shut down a factory! Require more strict mileage standards and require electric cars! None of this will reverse any coming severe climate change. Only geniuses may be able to save mankind now, if things get bad - and they are in painfully short supply.