Roosevelt once said:
"It is almost impossible for us in this day to realize how little our Revolutionary ancestors knew of the great store of natural resources whose discovery and use have been such vital factors in the growth and greatness of this Nation, and how little they required to take from this store in order to satisfy their needs."
which most likely means that to Roosevelt, the fact that the forebears didn't take many of the resources and that little they needed to satisfy their needs was one of the reasons for the conservation.