I don't know all the reasons, but some like when they've been in a clan in which a cheater got caught or sth, then they may shame on their name and don't want to let all see they've been in such a clan.
And it's their playernames, so if they don't want to share the name, we won't (like said) because we want to follow the users opinion. So if we will do sth like that, we will do it probably with some request.