It's because the moonlight is comparatively dim. Even when your eyes have fully adjusted there is a threshold below which you just can't pick up the light. Th

It's because the moonlight is comparatively dim. Even when your eyes have fully adjusted there is a threshold below which you just can't pick up the light. The scattered moonlight is below your detection threshold so the shadows appear completely black. Your color cones are also less sensitive than the black and white rods, so colors are muted or even missing and you are left with a landscape of stark greys just like the moon.