My answer, in terms of process:-

1) everything here is covered by several hundred metres of layered "white" rock

2) the "white" rock is very soft and fine grained, therefore it erodes away to nothing when dust-blasted

3) the "white" rock contains the dark spherules and nodules that weather out of it

4) the ground becomes covered by a lag deposit of spherules which armour the underlying terrain against further deflation

In summary, any chuncks of white rock blasted onto the surface get eroded away in a rather short time frame.

A suggestion for some research - pore through the MOC images of this region looking for signs of a fresh impact crater. See if it has a "white" ejecta blanket.