If the passenger side mirror were mechanical, it would require the driver to reach accross the car to adjust its position, thus it is much more convenient to make it electrical. I guess Mercedes thought that they could make the driver's one mechanical becuase the driver does not have to reach anywhere awkward to adjust it. Thus it would be very inconvenient for the passenger mirror to be mechanical, but they can get away with it for the driver side mirror.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record