Unlike small artifacts that can be excavated and moved to the safety of a museum or private collection, the very nature of cave and rock art exposes these cultural relics to the danger of weather, and worse, vandals. Preservation of such locations is a continuing issue that is not always easy to address.
On another note, if you’d like to explore areas of ancient rock art closed off to the public, I recommend you try out this virtual tour website of the Lascaux Caves, home of some of the most famous paleolithic cave art in the world. Sorry it’s in French, but you can pretty much get the idea visually through the virtual tour.
You can also visit another cave system famous for the ancient art on its walls, the Cave of Chauvet-Ponte-d’Arc (click visit the cave).