Mars doesn’t have a global magnetic field. So Daddy Sun spanked Mars off its atmosphere. Low atmospheric pressure = low boiling point for water (-4.96°C on the surface). Thus, all water got converted to water vapor, thus becoming part of the atmosphere, which was getting spanked… Sooo most water was lost. Most of the water remaining is in the poles as ice and maybe underground.

Atmosphere blew away and took it with it, I guess.