Consider that Mercedes Benz is (was) a German manufacturer, most of the car specifications were based on domestic or European demands. Both A/C and power steering were not that popular in Europe in the 60s and 70s. Sourcing components to build an option was always an issue. Suppliers were just not there.
The specific thing with P/S:
When Mercedes started using P/S, they needed to put together a system. They could make their own steering gear/box, but for the pump they looked at an existing manufacturer. P/S was a popular equipment for cars in the U.S. and there was an existing pump supplier: Vickers Inc. Mercedes Benz pursued Vickers to supply a pump and Vickers deviated from their earlier decision to pull out of the Automotive business and agreed to supply MB. They were also pushed to make the pump in Germany and they did install a manufacturing line in their factory which produced industrial hydraulics in the city of Bad Homburg, north of Frankfurt. Capacity was limited and Vickers could supply all needs as longs as P/S remained just an option.
In the seventies, when front axle weights got heavier due to a wider use of V8 engines in cars, MB decided to make P/S standard. Vickers did not have the capacity to meet the volume requirements and MB did not want to rely on one supplier, so ZF was enlisted to make P/S pumps. In the beginning, ZF used a "rotation group", the core of a vane type pump (ring, rotor vanes) sourced and later licensed from Saginaw Steering Gear in the U.S. , who got started making pumps with a license from Vickers.
