Actually, the cartoon infographic from that article offers a short and to the point explanation: "Giant impacts created oceans of molten rock that formed the da

Actually, the cartoon infographic from that article offers a short and to the point explanation: "Giant impacts created oceans of molten rock that formed the dark maria" ("Maria" being the plural of sea in Latin) - so the craters that were there before were actually molten, and any craters that can be seen were formed later and are therefore fewer than in older regions.