Why can't rangers protect "the nation's most valuable scenic, historic, and prehistoric treasures"?

Answer: Some of the treasures are in places where its almost impossible or dangerous to retrieve and to keep safe.

Explanation:

for example in the caballo mountains in new mexico there is aztec treasure. To get to the treasure you need to go through some cave systems. It could be dangerous because you could be crushed by rocks or lose your weigh and don't know the way out.