I've made several add-ons and not released them, usually because I'm not completely satisfied with the way they work.
Also large gamemodes should be kept private (IMO), so that the userbase can be kept on one (or a few) servers instead of being spread among dozens of servers.
Keep in mind that in many cases, these "private" add-ons are not even private (The mini-nuke and Ion cannon are two examples that come to mind). They were publicly released, but then failed because they were broken, released pre-v9 and never publicly ported to the post-v9 format, released on RTB2 and never downloaded from the archive and publically reuploaded before the archive was taken down, etc. In some cases, so-called "private" add-ons are still available in the add-ons section.
In these cases, it's because simple people like having something to gloat about.