General American English Pronunciation (GAEP) is a kind of standard pronunciation found in American dictionaries such as the Merriam Webster. (On the next page you will see the main characteristics of this kind of pronunciation followed by some examples.) We should not confuse Standard American English Pronunciation with Standard American English. The latter is the name for the grammar and vocabulary of the written and spoken variety generally used in the fields of education, law and government.
GAEP is not the only kind of pronunciation found in the United States of America. There are several others but the two main kinds are: the New England accent and the Southern English accent.
The English started to colonize America in the early 1600s. They came from different areas of the UK and and were from different social classes. The Puritans settled in New England and other settlers in Virginia. Those who settled New England mostly came from the South East of England and the London area and were from the middle and lower classes. The settlers in Virginia were mostly from the upper classes. Both these groups were in touch with England and had non-rhotic accents.
The reason why American English is generally rhotic is that during the colonization period many parts of England were rhotic, for example, the South West -as we saw when we looked at Cornish English. Also many settlers came from Ireland and Scotland which are also rhotic.