WikiTree was made invitation-only in January, basically to temporarily slow its growth a little bit. Here's a quote from Chris Whitten on why the change was made (from the press release):
“The problem is that we’ve had users abusing our wide range of free services and treating the site like their personal back-up service. Not only does this drain our limited resources, it hurts other community members and what we’re trying to accomplish. By making membership strictly invitation-only, we can give our community some breathing room to clean up the genealogical messes that irresponsible users have left behind, and we can be more careful about making sure that future members embrace our vision and understand our Wiki Genealogist Honor Code.”
Basically, the change to invitation-only was meant to decrease the number of users who just signed up, uploaded a GEDCOM, and then never responded to messages from other users, refused to merge profiles, etc.