Just guessing here :
I believe that the changes to the wiring diagrams that resulted in having the column cutout switches (the ones hidden in the control columns) work effectively for autopilot and STS but be ineffective for the MCAS signals pushed towards having a single electric channel feeding the STAB motor with a single cutout switch.
And removing one of the two switches was not desirable for keeping the FCTMs of the Max and the NG as close as possible.
Solution : 2 switches wired in serie. One is labeled "main" and the other "backup".