Not only will that art probably never get shown, but chances are also excellent that it will eventually be sold. Other reasons for not showing art are that museums have limited display space, curatorial changes affect what museums show, exhibition schedules are put together years in advance, some art has more research value than display value, and some art may be great, but out of fashion and not what the museum thinks the public wants to see at the moment. For all you potential donors, understanding the fate of your donations before you donate is your responsibility. When meeting with curators, ask direct questions, tell them why you want to donate, make all special requests before you donate, and, most importantly, keep your ego in check. Have curators explain their donation policies and tell you what the future holds for your donations.
All curators abide by strict codes of ethics and give the best explanations possible.
They usually have pretty good ideas of what's in store for individual works of art over the next twenty years or so. At the same time, be aware that no one can look far into the future and predict either the course of art history, how the scholarly community will eventually view particular types or works of art, or what your donation's ultimate significance will be. If you want your donation to have restrictions, tell the curators. You can ask that your art never be deaccessed, that it be shown within a certain time period, included in a catalogue, and so on. Institutions consider all such requests and either accept them, ask that they be modified, or deny them altogether.