Here's why:
* Maria are younger: The maria formed from ancient volcanic eruptions, filling in older, heavily cratered basins. This makes them relatively younger than the highlands.
* Lower elevation: The maria lie at lower elevations than the highlands. This means they are less likely to be hit by large impacts, which tend to target higher terrain.
* Lava flows: The lava flows that created the maria effectively "smoothed out" many of the craters that were already present.
While the maria do have craters, they are less numerous and often appear smaller and shallower than those found in the highlands.
