Why are the atmospheres of Venus, Earth and Mars so different?

We think it all has to do with their distance from the Sun and their masses. Venus, the closest, was too hot to support large bodies of liquid water. The carbon dioxide that was leached from the rocks into the atmosphere could never be returned to the rocks by water acting as a moderator. As the CO2 increased, so to did the greenhouse heating and it eventually ran away with itself, stabilizing at the almost pure carbon dioxide atmosphere we now see. For Earth, liquid water transported CO2 from the atmosphere back into the crustal rock, and its balmy distance from the Sun insured that no greenhouse process could easily run away with itself. As for Mars, it has so little mass that it could not hang onto its atmosphere heated by the Sun, and during the first billion years or so, it lost whatever it might have once had. All that remains behind are the heavier components to that original atmosphere including carbon dioxide.