Im sure in the 1700s the founding fathers had an english accent since their fathers and grandfathers came directly from England, but how did we lose that accent over time?
Can you be more specific? "English accent" can mean many things, since England has many different accents depending on which part of the British Isles someone is from. Even different parts of London have different accents. And when it comes to America, it's not just the south that has a different accent: Maine or New England, is slightly different from Boston, or Wisconson, Alaska, Seattle, Utah, etc.
You might be interested in this section of the FAQ. The short version is that while American settlers in the 1700s had the accents brought over from England, the variety of English accents was very different than today. Both America and England have undergone language change, neither one "lost" an original accent (or rather, both did, but as part of natural language change over time).