3rd style painting in Oplontis. The brick wall has been constructed to prevent the volcanic mud wall from collapsing.
I've never been excavating in Campania myself but this picture tells a lot about how it can be. Some areas, like Pompeii, were primarily buried by pumic stones - others by pyroclastic flows or mud.
How an area was buried determine how easy it will be to excavate it. The pumic stones are difficult to dig in as they keep on falling down in the trench again while the volcanic mud harden and turn into a concrete like material.
We also have to consider post-antique and modern constructions on top of the ancient remains. Thus we sometimes find ourself in a situation like this one: one part of the villa at Oplontis have been excavated inch by inch through a solid wall of mud (which preserved the spectacular decorations) until something at the modern ground level made it impossible.
This is simply how far we can get, even as we know for a fact that there are absolutely stunning finds just centimeters into the wall.