The Kennedy accent is to close Virginia and Maines each is easily mistaken for the others by everyone but native speakers. Accents from Virginia through Maine and parts of Canada all the way to the UK and South Africa are at least as close to each other as Southern ones are to each other (particularly, once again, the Virginia Piedmont accent that IS Southern, yet still sounds more like the others than it does ANY Southern accent.) Regional accents usually share a close relationship reflected in their sound, but speaking of the "Southern" accent as if it does not have several distinct forms is like speaking of the "English accent" as if there are not West Country, Welsh, Cockney and other varieties (which, to my ear, sound so much alike the differences are LITERALLY accents rather than phonemes, though the Scottish and, to a lesser extent, Irish accents are more distinct.)

It is not as simple as saying, "The most common English accent is that of Texas, Louisiana, Arkansas, Tennessee, Mississippi, Georgia and Virginia," because those are (at least) FOUR DISTINCT accents. To the extent such statements are valid it is in the sense of accent subgroups, which also include the Midwestern-Canadian-Pacific Northwest accents, "South by Southwest" accents and Virginia-New England-UK-South African accents. Further, there is overlap at the edges, especially in the US; the West Texas accent is closely related to the East Texas one, but hardly Southern; the Great Plains has elements of both its Southwestern and Midwestern neighbors; the Piedmont accent bridges the gap between the Southern and New England groups.

The GROUP spanning three continents likely remains the most commonly spoken, and not just because the North has long outnumbered the South >2:1. That is A reason; the Southern group may (or may not) cover more LAND, but one need only look at an Electoral College map to see how little area corresponds to PEOPLE. Again, had the North not outnumbered the South >2:1 it probably would not have won the Civil War, but it did, so it did. Yet, (geographically and otherwise) beyond that there is a reason the aptly named Middle Atlantic accent was so heavily promoted on both sides of the Pond, leaving most Americans unsure whether folks like Cary Grant and Katherine Hepburn were Virginia aristocrats, Maine shipwrights, British lawyers or South African plantation owners: Broadcasters and feature film personalities needed an accent that felt natural to the bulk of the English-speaking world.

Their choice of necessity was no more Southern than anything else, and decidedly less so than it was British. The sun never set on the British Empire; it never shone >18 hours on the South.