Actually, it is partially a modding issue; places like Schnoogle are updated by hand by the mods, and it can take some time. Sometimes people won't post there until it's done, which is a practice I like. :)

Other times it is a revision thing - they might want to be able to go back and revise, which is harder at moderated archives, so they only post to places where they control the story until they're pretty certain they won't be changing anything in a particular chapter.