Because we say so. It's an arbitrary rule; but on the Internet it very often happens that where people are allowed to post anonymously the privilege is abused.
Other people have moderated your posts, and more have been moderated down than moderated up. As the initial rating of your posts depends on how well your peers (the other users of this system) rate you, this means that although you can still add posts to the system, they won't be seen by anyone. If you think this is unfair, you can ask the administrator to reset your rating.
Pages on this site are held in a cache and regenerated from time to time (by default after 20 minutes). So your post won't be visible on the site until the next time the cache has regenerated. Be patient, it'll get there.
If your post has been moderated down by another user (or a post it responds to has been moderated down) to such an extent that it's rating is less than a preset level, it won't be shown.
That's too bad. From time to time the administrator of the system will give you moderation points, but once you've used them you've used them.
You can't.
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