Reasons that I hide my identity on the net:
(Note: a fixed nickname is differ from hiding identity.)
1) Nicknames are fun, can change it whenever I want.
2) Real name represents my outer shell. When I take off that shell, I don't want others to hurt me or being hurt.
3) Don't want others recognize me - simple.

For people has interest in reading public blog and forum, they don't care who you are, they get what they want and go. At that time, your name is just an invariant.

4) So like the Parable of the Sower,"they may indeed see but not perceive, and may indeed hear but not understand".
5) Sometimes just NO need to let others know who I am.
6) Could be lacking confidence or avoiding responsibility.