The reason it is “easier” to land on an airless body as opposed to Mars is counterintuitive.

The thin Martian atmosphere is almost a worst-case scenario. When a fast-moving spacecraft encounters the atmosphere it experiences substantial heating, but doesn’t receive much benefit in the way of speed reduction. Landing on an airless body, like the moon, a spacecraft just has to manage its relative velocity to the surface. Landing on Mars, a similar trajectory would be mathematically possible, but results in massive friction heating issues.

If there were no air on Mars, landing on it would be much simpler. If there were a lot more air on Mars, landing on it would be much simpler. Unfortunately Mars’ atmosphere is just thick enough to be problematic from a heat standpoint and not thick enough to be optimally useful.

The entry, descent and landing system the Curiosity series probes uses is heavy. It weighs over 2k kg (over 5k pounds). If Mars had no atmosphere, most of that weight could be replaced with a simple rocket system like those used to land on the moon. The LM used in Apollo had a fully fueled weight over 15k pounds, and that included the ability to ascend back into lunar orbit. But a landing on Mars like an Apollo lunar landing is practically impossible due to the atmospheric heating.