1) The colonists broke into FOUR distinct migrations from different parts of the British Isles, and each having different "cultures" (or "folkways"), which included different dialects, not the same one. And those foundational divisions, though now modified by other events (including the migrations of NON-English speakers).
2) The dominant accents of modern British English (and esp the "Received Pronunciation") was NOT yet dominant at the time of the American colonial migrations. So Americans did not "lose" such an accent -- they never HAD it!
3) Also, MANY of the things (including accent) that distinguish the main British dialects from the American did not yet exist in colonial times (so again, Americans could not have "lost" them!). The "dropping" of final r's is a prominent example.
This is a key point many miss -- both American and British English have changed over the past four centuries, but it is especially humorous to hear a pro-British complaint about some change Americans made in the language, only to research the matter and find that Americans in this case are PRESERVING something older, while the British changed it!