Hi Joe, once a museum has an item, they are the legal owners and can do with it as they see fit with it. In many cases this means sell it. If the item is not what the museum wants, ie their specialist subject, they often have no real interest in it. Things also go "missing" ......They are under no obligation to display an item. I know Vets who have donated items which have never seen a display case. Also some storage conditions in some stock rooms are terrible. As is the way they catalog items.
I have been behind the scenes in many UK and European museums and on the whole I am not impressed.