Jorge - Some museums are pretty good. The Center for Creative Photography here allows anyone off the street to sign up for a print-viewing. You can see original prints by Adams, Siskand, Frank, Ulesmann, etc.
That said, they do pull stuff from public view. They just pulled all their Arbus prints because of concerns regarding conservation. And since conservation is insanely pricey, you have to be careful. They spent something like $25,000US to have a conservator come in, examine all their Edward Weston prints, digitize them (just in case), and then they could only preserve THREE....yes, THREE prints!
So I can see why keeping stuff inthe nice humidity controlled room in the back can be tempting....
At the same time, museums are about showing the public things.