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.
One of the primary theories explaining why ancient Mars was more humid than today is that a dense, carbon dioxide-rich atmosphere created a global greenhouse effect, warming the surface sufficiently to allow large amounts of liquid water to exist. The limited volume of carbon dioxide ice in today's polar caps is insufficient to support this ancient atmosphere.