My guess: as you will find with all aircraft with a some form of autopilot, there is a requirement for a disconnect in case the system fails. Even though it is dual channel, things can go bad. Imagine engaging PAC, letting go of the trigger, but it remains engaged, or half way through a turn the SAS faults and engages full rudder for your turn coordination. There always needs to be a quick disconnect ready at hand for safety, as things will go very bad, very quickly.