ну америка очень большая и бай дефолт состоит из микса иностранцев. а в британии тысячеления эволюции языка и до сих пор не существует института который бы положил на бумагу единый стандарт
возможно причина в том что диалект был способом различать социальные классы
For historical reasons dating back to the rise of London in the 9th century, the form of language spoken in London and the East Midlands became standard English within the Court, and ultimately became the basis for generally accepted use in the law, government, literature and education in Britain. The standardisation of British English is thought to be from both dialect leveling and a thought of social superiority. Speaking in the Standard dialect created class distinctions; those who did not speak the standard English would be considered of a lesser class or social status and often discounted or considered of a low intelligence.[27] Another contribution to the standardisation of British English was the introduction of the printing press to England in the mid-15th century. In doing so, William Caxton enabled a common language and spelling to be dispersed among the entirety of England at a much faster rate.[11]