In a few words, Modern Radios.
The "rules" were developed when we flew AM and ignition.
There many things that will reduce the safe range of your plane. You can fudge a few, but sooner or later you will be some place and the "fudges" are going to gang up with other things and plant your plane. The idea is to make the safest installation to give you the most headroom and keep you flying durning those times when the forces of evil gang up on your plane.