Why has the surface evolution of the Moon and Mercury essentially stopped?

The surface evolution of the Moon and Mercury has largely halted primarily because they have a small size, which limits their ability to retain internal heat. Smaller celestial bodies cool down more rapidly than larger ones due to a higher surface area-to-volume ratio, which leads to the dissipation of volcanic activity and other geological processes over time.

As these bodies cooled, they stabilized, resulting in a lack of significant geological activity that would otherwise contribute to surface evolution. Their small size means that they do not produce sufficient heat from radioactive decay or maintain tectonic or volcanic activity, which are critical for surface changes.

In addition to this, the Moon and Mercury have experienced limited atmospheric influences in their geological history, which also contributes to the stagnation of surface evolution. However, the primary reason for their current state is indeed their inability to retain heat effectively due to their size.