For many years Bernstein explicitly refused to place any sort of license conditions on most of his software on the grounds that he didn't want modified versions

For many years Bernstein explicitly refused to place any sort of license conditions on most of his software on the grounds that he didn't want modified versions floating around and copyright law allowed most of the things he wanted to allow people to do with his programs. (It's unclear whether or not he was right on the latter point. You can dig up Bernstein's flamewars with Theo de Raadt and Rick Moen on the topic and decide for yourself.) I don't think he gave a reason why he switched to public domain a few years back, but it was clear by that point that he wasn't doing any more work on qmail or djbdns and the non-licensing was holding back further development and adoption by others.