

Manya's grandfather organized the wastelanders into dragging scrap metal from a destroyed airport to set up the walls around the rim of the crater, which means they had a larger vertical profile to build shacks. The crater (which was actually created by a crashing jet aircraft, not the bomb) provided the best natural shelter of anywhere nearby, against both raiders and dust storms.

So when megaton was first founded, which seems to be about 120-ish years ago, the Capital Wasteland was worse off than it is now.
