Why do museums keep most of their collections out of sight?

Collectors have a hard time understanding that museums are not just a place to display to the public rows and rows of antiquities, and that anything in the reserve and research collections is somehow 'unnecessary;' and can be deaccessioned and sold to them (so they can 'display and preserve' them, act as their gatekeepers and brag about them). Kimberly Bradley, ' Why museums hide masterpieces away' BBC 23 January 2015) addresses this problem from the point of view of artworks.

At New York’s Museum of Modern Art, 24 of 1,221 works by Pablo Picasso in the institution’s permanent collection can currently be seen by visitors. Just one of California conceptual artist Ed Ruscha’s 145 pieces is on view. Surrealist Joan Miró? Nine out of 156 works. The walls of the Tate, the Met, the Louvre or MoMA may look perfectly well-hung, but the vast majority of art belonging to the world’s top art institutions (and in many countries, their taxpayers) is at any time hidden from public view in temperature-controlled, darkened, and meticulously organised storage facilities. Overall percentages paint an even more dramatic picture: the Tate shows about 20% of its permanent collection. The Louvre shows 8%, the Guggenheim a lowly 3% and the Berlinische Galerie – a Berlin museum whose mandate is to show, preserve and collect art made in the city – 2% of its holdings.

The main reason for this is a spatial deficit, most institutions do not have the space to show more in a manner palatable to the modern visitor. Another reason is fashion: some holdings no longer fit their institutions’ curatorial missions, but a museum stores memory, or culture, and these items are carriers of those values whether or not we like to look at them. Some works stay under wraps due to delicacy or damage and await expensive conservation care. Other things are too delicate to have on long-term display and exposure to light and the ambient environment in display galleries (a prime example is Albrecht Dürer's famous watercolour and gouache drawing Young Hare of 1502 discussed in the article with a link to the amazing 'Google Cultural Institute Gigapixel image' - check it out). Another example given is   Henri Matisse’s The Swimming Pool (a large paper installation made for the artist’s dining room in Nice, are in fact currently on view in the exhibition Henri Matisse: The Cut-Outs until February 10 at New York’s Museum of Modern Art). Several other works are discussed with various reasons for keeping them in store. Disappointinly the use of reserve collections for research purposes is not discussed.