During the formation of the gravity well that is earth at this particular distance from our star, water would have been attached )adsorbed) to the minerals and ‘star dust’ which was aggregating to form earth, plus the orbital pathway earth travels would have been ‘swept’ by the orbiting planet as it formed. If you look at the materials in abundance on the various planets, it appears that Earth and Mars and the asteroid belt are in a zonal distance from the star which is water rich, so to speak. Why, for instance, does earth have so much water and Saturn does not? And why are certain sized moons richer in water than other moons orbiting the gas giants?