You can pressurize a chamber using the thin atmosphere, rather than being restricted to the gasses you bring with you. You can use ISRU and atmosphere resource

You can pressurize a chamber using the thin atmosphere, rather than being restricted to the gasses you bring with you. You can use ISRU and atmosphere resource extraction to generate rocket fuel on the surface of mars, rather than being restricted to the rocket fuel you bring with you. Ultimately, the answer is Mars has the necessary resources and when you look into the details just makes more sense and is easier.