The Current QB Passer Rating System

The current QB Rating System for the NFL was invented in 1973 by Don Smith.  It is supposed to measure a quarterback's passing prowess.  It's a metric that uses 4 statistics to come up with a number that is supposed to represent the quality of a quarterback in terms of his passing ability.  The following are the four statistics used:  

1) A component, [C] for Completion Percentage, as defined by ({Completed Passes/Passes Attempted} * 100 - 30)/20

2) A component, [Y] for Yards per Attempt, as defined by ({Yards Thrown/Passes Attempted} - 3)/4

3) A component, [T] for Touchdown Percentage, as defined by ({Touchdown's Thrown/Passes Attempted} * 20)

4) A component, [I] for Interception Percentage, as defined by (2.375 - {Passes Intercepted/Passes Attempted} * 25)

Here's how the four statistics are used in the calculation of the metric:

QB Rating  = [max{min(C,2.375),0} + max{min(Y,2.375),0} + max{min(T,2.375),0) + max{min(I, 2.375),0}] * 100 / 6

This guarantees a QB Rating between 0 and 158.33.  

According to the Pro Football Hall of Fame, the NFL has historically changed the "Passer Rating" system a total of nine times.  However, the current Rating System has remained in place for the past 35 years.

All of which begs the question, why?