Umm... The main cause the Publishers give more often than not is, the book was not successful to warrant a second run and hence it's out of print or 'it's just old'. I just read this question which claims there may be other motivating factors for a publisher to not print and still try to hold over rights.
Are there any other such factors which Publishers use or could use to not give royalty payments to authors as well as not let public know about authors or their books ?