I remember being told that it had to do with power -- early 12V systems just could not run it all unless you made the charging system to large.
Vac power is quite strong for its size and especially weight and it is very simple -- just think -- in the vent system you do not need any limit switches etc.
My problem with mercedes is in some cases they stuck with it too long -- door locks and especially trunk and gas door.
I think I hear motors in my Jag and BMW ACC systems from time to time. I do hate the Benz ACC Vac systems -- But then again the Benz window motors used to go all the time -- so may be we are really lucky they did not go with any more motors.