The answer might not be so obvious but: English was brought to North America by British colonists in the 17 - 18th centuries. 
By the time the huge British migrant colonization began, a significant number of Indian peoples already lived here - all of them were speaking autochthonous languages (around 37).
The English and the Indian languages had mixed but the first one was dominant. 
And it turned out that English became difficult a bit for native citizens on North America so they started mispronunciating some words with complicated sounds such as: "apologiSe VS apoligiZe".
And people could not find a common language with the British colonists and found their own English - American.