You can't just cover things back up. Archaeological digs have been slowly buried over time in environmental conditions that allowed for their preservation, or in Pompeii's case, initially very quickly and then slowly. Covering it back up would not only ruin the discovery potential of future investigation that relies on identification by context (for example, dating a pot by the chemical composition of the surrounding and previously contained materials, but it would also endanger anything we've found by introducing an uncontrolled and entirely new environment. It's not like we can layer on the ash and other stuff in the same order it was deposited and in the exact same location with the same chemical composition.

Conservation is a necessary and very active effort as soon as something is found, because the act of studying it aleays causes at least some initial destruction.