Answer:
First it must be said that when the first British settled on the east coast of the current United States at the end of the 16th century, there were already Europeans in this region. Cocretely French, Spanish, German, Dutch and, very importantly, Welsh, Scottish Irish. Most of them had arrived on Spanish and French expeditions that established colonies in the Florida and Louisiana area. It was then in 1583 when Queen Elizabeth I of England authorized the pirate Sir Walter Raleigh to establish the first English colony in North America, specifically in Lousina and then extended to Maine, in the northern United States. From this moment, British English began to change slowly due to the influence of both French and Spanish as well as English spoken by Welsh, Irish and Scots.
From this influence, American English is one of the most important differences in pronunciation with respect to British English: the pronunciation of r and t.
As for the pronunciation of r, in the seventeenth century it was also pronounced in England and, for example, sister or brother were pronounced with the sound of the soft r included. Today, these two words in England are pronounced as sista and brotha. On the contrary, American English has kept the sound of the r - between syllables and in the end - and other words like art, which British English are pronounced with an elongated, in American English a soft r continues to sound.
Also the consonant t is another classic example that differentiates English pronunciation from British. Words like bottle teach us that the t among syllables in North America is pronounced as a d (boddle), while in British English the sound of t is loud, like tch when pressing the tongue between the teeth.
