Many are just copyright "orphans", nobody knows who owns the copyright any longer. Maybe the author died and the copyright passed to their estate, but they're not even aware of it.

One book I'm hunting for a copy of right now was published in England in 1947 and in those days paper was rationed, so not many copies were made, and only a handful have survived. As soon as I find it I'll scan it and upload it to IA.