The "posting" approach gives everyone a chance to read messages at their own speed and leisure.
You are correct: the chat room provides for a more immediate communication. However, the current chat room technology does not permit the same level of communication permitted by the board format. I've also noticed that the chat room seems to have a heavy social emphasis. Even if chat room conversations were "saved" for others to read, I don't think I would care to sift through the chatter to find the answer to a question.
