ELI5: What are the roots of American accents? Where did the English accents go?

Specifically I'm wondering how the typical English accent became the typical western accent (Which sounds relatively country), and how did that become the modern accents on the West Coast? What factor was added in that made cowboys start sounding like the modern day Californian.

I'm assuming the typical NY accent comes from Italians coming over.

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Bonus question: Why are there no places in the US at all that kept the English accents????