Boeing may have considered the reliability of the automatic system or other failures associated with a need to abort that would result in not having an automatic system. Similarly, for those automatic systems that have to be armed, the manufacturer or operator may choose to consider any crew error in failing to set the system.
Then of course, there is the possible error in monitoring.
If in doubt ask the manufacturer for a detailed explanation; there may have been a change in the system design for new variant aircraft, or even Boeing may have made a mistake.