Answer: because of the eruption of Mount Vesuvius
Explanation: the volcano that destroyed Pompeii basically incased the whole city lava and when that lava cools you get volcanic rock and that would preserve the buildings and the people were also incased in that rock as well as ash the city was under 20ft of rock so the wind and rain could not wether or destroy anything neither could mold or bacteria get on the food everything was basically sanitized and then fossilized