Why did the United States, which was mainly established by Europeans, form a unique American accent?

America is a country of immigrants. It can be said that there are descendants of European citizens in the United States, most of whom are British, so English has gradually become the mainstream, but at the same time, it has formed American English with other European integration of language. English as the main body, slightly improved. It's a bit like the Shanghainese dialect spoken by Shanghainese now, which is different from the old Shanghainese dialect, and it combines foreign words such as Japanese, English, Spanish and Mandarin.