Because everyone beleives their views are better, or they have a bigger stick .
For instance the USA often refuses ICAO when it does not suit them :
a quote from a former FAA top executive during a famous meting on " line up and wait" : "we have 600.000 pilots and 20.000 controllers in the USA following good sound FAA procedures, you want me to tell them to change their ways because a UN organisation is saying so ? "
And that is the biggest problem we face. National stupidity. My answer to the above is that 25% of the world population is chinese and within a few short years they will have more pilots and controllers than the rest of the world put together, then throw in the Indian sub-continent. That kind of attitude from the FAA shows that they are incompetent to have a big say in how things are run because time and again the USA demonstrates its shameful lack of knowledge and understanding of the rest of the planet. The same applies to the UK CAA, only interested in national issues.
Lets just have one standard, one document and everybody stick to it.