But, I am willing to give more details about how the line came about by having you look at a map below.This map compares the current divide amongst sports teams in red (Philly) and blue (NY/ NJ), to the first line dividing New Jersey called the Keith Line which was not north and south, but EAST and WEST.
Well before it became a state, in colonial times, New Jersey was divided. There was an East Jersey and a West Jersey. And the first line drawn by Surveyor General George Keith, only made it up 2/3 of the way before we realized he drifted too far west and stopped near the Raritan River branches. The line in black represents that line and continues west, if Keith had finished it. Just look how that original line, which still demarcates the borders between Ocean and Burlington, western Monmouth and western Somerset, matches the Sports' loyalty line. From 1686 to 2008, it is amazing how little some cultural things have changed.