First impressions/best foot forward. No one in the congregation wants to give a visitor the impression that they are anything but united and loving and a dignified representation of Jehovah's people. Otherwise, God might feel that you let him down.
Once they're in, you can relax a bit, but you still tell yourself that you have not changed your behavior. That would be dishonest, and God wouldn't want that! And once they're in, the new members subconsciously follow everyone else's lead, so they are ready when the next visitor arrives.