I expect that Dune is much, much more common in libraries than any of its sequels, or at least is checked out more often.

This is supported by a quick search of my local library catalog, which reveals that the library system here has zero to two copies of any given Dune sequel, nearly all of which are currently available, but six copies of Dune, only one of which is currently available.

The other library I sometimes visit appears to have zero to one copy of each Dune sequel, nearly all of which are currently available, but four copies of Dune, zero of which are available.

Obviously, this is a limited sample, but I expect that similar trends generally prevail.