Because the proper operation of the diesel prime-mover, in an actual application, requires quite a few inputs and outputs. Some of the inputs reflect requested speed, but also the actual load on the engine. By balancing all of these many factors, the proper control of the engine becomes possible. The governor is actually a pretty complex analog computer which, in today's engines, would be replaced by a digital computer called an engine control unit.
I LOVED toward the end about flushing out the unit with "carbon tetra chloride." This is considered to be a dangerous chemical with serious health (including cancer) risks. Up to c. 1960 it was still used as a fire fighting fluid! Now it's close to impossible to buy.
