A couple of possible reasons: 1. Some folks are getting fed up with Ancestry and moving elsewhere, and taking their 'trees' with them. They aren't particularly eager to provide free content for Ancestry to use in helping market its product. 2. Other folks are staying with Ancestry but making their 'trees' private because they are fed up with raiders who lift their content, often using it incorrectly, without ever bothering to say thanks or to engage in collaboration and exchange of ideas.