* Age: The maria are younger than the highlands. They were formed by volcanic eruptions that filled in large impact basins billions of years ago. This means they had less time to accumulate craters compared to the older, heavily cratered highlands.
* Lava flows: The lava that formed the maria covered and smoothed over existing craters. The smooth, dark plains we see are essentially giant, solidified lava flows.
* Lower elevation: The maria are lower in elevation than the highlands. This means they are more likely to be shielded from impacts by the highlands, which act as a kind of "shield" against incoming objects.
In short: The maria on the Moon are younger, smoother due to lava flows, and slightly protected by the highlands, all contributing to their relatively fewer craters.
