[RightWhale]

Probably because of the danger that the ash presented to later scavengers

Could the ash set up like concrete quickly? Seems like Pompeii was not loose dust, but fairly hard material. Shovels wouldn't do a lot of good.

[Quix]

I believe the issue is that the ash compromised life through the lungs. Sluggish and slowly dying animals collected at the watering hole and were buried by more windblown ash.

It seems to take a while to compact if my Spokane experience is a clue.