Do you think this is because many BookCrossers specifically say in their release notes that people can journal anonymously without joining? Would it be better to explain that you can, but then you won't be notified of the rest of the journey?
A while back I edited my standard release note to say, "If you make a Journal Entry or Release Note on the book after joining BookCrossing, you’ll be alerted by e-mail each time someone makes another journal entry, so you can follow this book's travels." And "(You can make a journal entry anonymously if you don't want to join, but you will not be alerted to the book's further travels.)"
I have no way to know if that changed the number of people who joined or not. I suspect people might not even read the Release Notes at all.
Just a thought and wondered what others feel about this when I see the comments about not joining so often here.