Why don't long-term liquid water deposits exist on the surface of Mars?

Long-term liquid water deposits do not exist on the surface of Mars primarily because the atmospheric pressure is too low. Mars has a very thin atmosphere composed mostly of carbon dioxide, with surface pressures less than 1% of Earth's. This low pressure means that water cannot remain in a liquid state for extended periods; instead, it tends to rapidly evaporate or freeze.

At higher pressures, water can exist in a liquid state at a wider range of temperatures. On Mars, the conditions are such that even if temperatures rise sufficiently for water to appear on the surface, the low atmospheric pressure causes it to transition to a vapor state almost immediately, preventing the formation of stable bodies of liquid water over the long term.

Other factors, like temperature and radiation, are certainly challenges for water retention, but the fundamental issue is the inability of the atmosphere to sustain liquid water due to its low pressure.