Because John Hospers doesn't have a web site, and because the letter just went out a few hours ago. But you can find the letter, since I last checked a couple of hours ago, on www.Solohq.com and on http://bidinotto.journalspace.com -- with many more to come. The owners of both sites are libertarians.
The letter is probably just being reposted b/c they saw it here. I also wouldn't trust the johnhospers@aol.com email address mentioned in this thread either. Hospers is a Senior Editor at Liberty magazine and a Professor at the University of Southern California. I would trust a contact from there more.