One of the hypotheses is that the United States borrowed the way it was written from the United Kingdom who used it before the 20th century and then later changed it to match Europe (dd-mm-yyyy). American colonists liked their original format and it's been that way ever since.
Is the US the only country that uses mm dd yyyy?
Despite the variety of date formats used around world, the US is the only country to insist on using mm-dd-yyyy.
How do Americans write their dates?
When you prefer to write the date in American English, usually the month comes before the day, then followed by the year. If we use the same example as before: The 6th day of the month September, in the year 2019, then the date in American English should be written as: Sept 6. September 6.