The data show two separate trends.
For in-copyright books, the publisher's initial printing sells out and when sales drop off (for paper editions) the publisher stops printing more copies. Readers buy used copies, easily available from Amazon, Alibris, etc., which have taken over the market for older books, so eventually no new copies are available. If lots of copies were printed, the volume of used books is large and copies are cheap. If few were printed and more people want them, used copies are hard to find or expensive. Readers may have a problem, but it's not as bad as it looks. Authors and publishers have more problems because it's harder to make money selling new reprints.
The large volume of new copies of out-of-copyright books probably comes from the ease of print-on-demand publishing. A simple recipe for do-it-yourself publishing is to set up a book template, skim the texts of public-domain books from Project Gutenberg or a similar source, and put the books into the system, without investing a penny in paper or ink until someone buys a copy on-line. Look up a public classic, like The Deerslayer by James Fenimore Cooper, and you'll get dozens of different new editions on Amazon. Go a little ways down the list and you'll start finding publishers you've never heard of. They (probably) are POD editions that can be cheaply produced. Because the investment is so low (compared to actually printing copies and distributing copies the old-fashioned way), there are many more editions of these old books available than newer ones -- but you'll never find most of them in a bricks and mortar bookstore.