One of the leading hypotheses for why ancient Mars was wetter than today is that a thick, carbon dioxide-rich atmosphere created a global greenhouse, that warmed the surface enough for liquid water to occur in large amounts. Carbon dioxide ice in today's polar caps is too limited in volume to hold that ancient atmosphere.
A leading explanation for Mars' past wetness is that a thick, CO2-dominant atmosphere created a global warming effect, enabling substantial liquid water presence on the surface. However, the current polar ice caps' limited carbon dioxide content cannot sustain this ancient atmosphere.