It doesn't have the mass or magnetosphere to maintain its atmosphere. The lack of mass makes it easier for the atmosphere to evaporate and be lost to space, while the lack of magnetosphere made the solar wind blow much of the atmosphere away. When Mars lost its atmosphere, it lost its water.
ceesa t1_iwnsf3y wrote
Reply to comment by Dangeresque2015 in Early meteorites brought enough water to Mars to create a global ocean — Meteorites bombarding the Red Planet may have carried so much water that it could have covered the planet in a layer 300 metres deep if spread out, while also depositing molecules essential for life by marketrent
It doesn't have the mass or magnetosphere to maintain its atmosphere. The lack of mass makes it easier for the atmosphere to evaporate and be lost to space, while the lack of magnetosphere made the solar wind blow much of the atmosphere away. When Mars lost its atmosphere, it lost its water.