Why? Mars doesn't have a magnetosphere. Until this is remediated, any atmosphere will be whisked away by the solar winds.
Venus has a magnetosphere. We could and should start not terraforming Venus, but "atmosphere-ing" Venus so that it can then be terraformed. Develop bacteria that live in the Venusian extremes, eat sulfuric acid and output fixed sulfur and H2O. Let the process run. We can then handle the rest.