Moreover, the availability of pictures of Mona Lisa is a big part of its popularity. Wouldn't you rather own the only copy of a famous atlas, then an obscure one?
They want to have something that others can't have.
>Wouldn't you rather own the only copy of a famous atlas, then an obscure one?
Some would, but there are plenty who prefer the obscurity. Most aristocrats are more interested in impressing their friends and immediate circle with off-beat style than in starting a popular museum or being seen as gaudy. Obscurity is hip--and sometimes it can even be affordable.