Why is Mercury so heavily cratered?

Mercury’s surface is covered with craters because it has almost no atmosphere to slow or destroy incoming meteoroids. It also has no active plate tectonics or strong weathering to erase old impacts, so the scars remain for extremely long periods. Many of its large basins formed during the late heavy bombardment more than 3.8 billion years ago. The planet’s slow cooling left a geologically quiet crust that acts like a record of early Solar System collisions.