I'm not sure why others might choose to keep their tree private, but it is allowed and it is a point in the Honor Code that we respect privacy. Privacy is, of course, required for living individuals. Certain levels of privacy are allowed for the recently deceased. Only profiles for people born over 150 years ago or who died over 100 years ago are required to be open. You can usually contact a profile manager to privately discuss any "private" profile or their tree which they may have private, bearing in mind it is their preference if they choose not to disclose information about that which they prefer to leave private.