Non-renewed copyrights: until recently, holding a copyright in the US required registration with the copyright office. If you failed to do so, no copyright for you! And if you registered, and wanted your copyright to last longer than 28 years, you had to renew your copyright after 28 years. The books referred to above were not renewed, and are therefor in the public domain.
Project Gutenberg (PG) has only recently started republishing these books in earnest, because it is harder to prove their public domain status. Instead of relying on the information printed on the title page alone, the project needs to be able to rely on the records of the Copyright Office too.
As with most of PG’s e-texts nowadays, most of these novels have come through Distributed Proofreaders. If you would like a sneak preview of your favourite fifties sci-fi, why not proofread a few pages?