First of all, you have to acknowledge there is a finite area for proper display. Secondly, this happens more in the artifact world than the fine art world. Third, not all parts of a collection are as good or even ready to display. Some are in need of restoration. Some are inferior to others on display. Lastly, museums like to rotate displays to help visitors see something fresh. All this doesn't mean that museum storage areas are not interesting. The Smithsonian has a very interesting one which I was lucky to lost in when I was a child.