alt Hacker NewsIIRC the first explosion of 79 AD didn't bury the Pompeii completely. (It did bury Herculaneum, and much deeper so.) It was another explosion around the time of collapse of the Western Roman Empire that finished the job and hid the remaining structures from human view.
The 79 eruption buried the first floor. Upper floors (if they existed) were still visible. Over time the upper floors were demolished to scavenge the stones to build other villages in the area. So when the site was "rediscovered " in modern times it was the first floor that could be excavated.