On Earth it took something like a billion years after photosynthesis evolved before there was any more than trace amounts of oxygen in the atmosphere - oxidation stripped it out of the atmosphere just as fast as it was produced, until there was nothing left to oxidize.

Meanwhile, as others have said, oxygen is one of the most abundant elements in the universe, and makes up a huge percentage of the mass of all rocky planets (ballpark of 40% based on available samples) , it's very possible that Mars was well and truly oxidized just as soon as the surface cooled down enough for stable oxides to exist. Heck, the proto-planetary cloud that eventually became Mars was almost certainly already incredibly rich with oxides long before the planet formed - but the heat of formation would have driven off the oxygen from all but the most stable oxides until temperatures cooled again.