As mentioned by Dan Mills, the standard RC hobby servo control signal dates from many years ago.
The signal period of 20ms (50 Hz) was chosen simply because it was a convenient number that fit the technical requirements. Each control channel takes a maximum of about 2ms. There are several control channels - the most that I've ever seen is 6 channels which can occupy a maximum of 12ms to 13ms. The remaining time is used as a sync pulse that synchronizes the receiving decoder to the transmitter multiplexer.
Thus: the signal from the transmitter is a repeating sequence of channel information (of varying widths) followed by a sync pulse that occupies the remaining time in the 20ms frame.
This allows for a very simple decoder / demultiplexer at the receiver: a simple shift register. A digital "1" is shifted into the first bit (only the first bit) of the shift register. The leading edge of each received pulse walks that "1" down the length of the shift register. The width of the pulse at each output of the shift register is thus controlled by when the start of the next pulse is received.
The Signetics NE544 is the first IC that I am aware of that was designed to control hobby servos. I still have hard-copy Signetics data books that go into great detail about how the whole RC control system was designed and how it works. That information is most likely still available online.
Pounding "Signetics RC Servo" into Google brings up the construction manual for a RC Servo kit from Ace: Ace Servo Kit Manual which has a bit more detail.
The bottom line is that the current PWM control signal that hobby servo motors use was designed long ago. The fact that it remains a standard to this day shows that the design of the protocol was sufficiently robust to remain a standard.
One final thing to keep in mind: the RC servo signal was primarily an analog signal in that the width of each channel's pulse was derived from monostable timers in the transmitter. The individual potentiometers in the transmitter simply adjust the width of the output pulse for that channel as each channel's timer was triggered in sequence.
The closed-loop electronics in each servo was also analog in nature: the feedback pot on the output shaft of the servo controlled the pulse width of a monostable timer. The servo electronics simply adjust the position of the feedback pot to match the pulse width of the timer to that of the incoming pulse. As the feedback pot is adjusted, so is the position of the output shaft.
Ingenious and very-well thought out techniques!