Stable orbit is both a velocity and an altitude. Orbital velocity is dependent on orbital altitude. The absolute minimum before Earth's atmosphere interferes too much is 160 kilometers altitude at 17700 mph. Higher altitudes mean more velocity is needed to be in orbit.

Not only that, Mars is on the outer edge of the goldilocks zone with a slightly oblong orbit, which takes the planet farther out of the zone for half of its year. If Mars had an atmosphere 343 days of its 687 day year would be extremely cold globally. A Mars with an atmosphere would be too harsh for complex life.