Because it fundamentally changes the game and you are at least 30 years too late for changes of that magnitude. Most sports have a period where their games rules are changed massively, Rugby League had that period from 1908 until the 1970's/80's. After that period of change sports tend to settle into the rules that they have, look at Rugby Union, Rugby League, American Football, they all developed from Soccer and had massive change periods that slowed as their audiences became accustomed to what the game was. It is easy to change the game when it is new and young.
Look at Baseball and Cricket that both evolved from rounders, again they had periods of huge change that then slowed as their audiences became accustomed to what the sport is. Yes cricket has introduced different lengths of matches but the fundamental rules remain the same.
Ultimately Rugby League is beyond its time of fundamental change and the only changes to come from here will be superficial changes to rules or changes related to safety, that is not necessarily a bad thing.