My reasons weren't listed either.
I want my posts to stand or fall on their own merits, not my personal reputation, for good or for ill.
People sometimes risk falling into a 'cult of personality' mentality. Too often, I feel that the message gets overshadowed by the messenger. I'm not happy with this state of affairs.
I post anonymously in an attempt to work around this quirk of the human psyche, and try to get a more honest reponse to my words. It's interesting and challenging; in my experience, anonymous posters are viewed as something of "second class citizens"; and I guess I've always been a fan of privacy, and of underdogs.
I've noted that the exact same posting seems to generate a better response if made under a newly invented pseudonym than if posted as the "anonymous" user. I haven't figured out exactly why, but the social behaviour interests me...