Because water (Hydrous minerals & water ice) and other organic molecules/substances had been found on mars and some other moons. These are the building environments for life. For example on earth, unicellular organisms and other like archaea can survive in very extreme conditions. In some places, life can survive in near-boiling heat or near-freezing cold, high pressure or high salt, as well as environments steeped in acids, alkalis, metals or radioactivity. If life can survive those extreme conditions, it is reasonable to think that life exists on mars & other moons.