Why did the creators of the game Pandemic decide that the starting position for each game would be Atlanta rather than any of the other possible cities on the board?
EDIT: I just checked the English and German rules and now understand your confusion. The German rules explicitly say that this is why players start in Atlanta. For some reason the English rules skip that part.
Which roughly translates to:
Atlanta is US headquarters for disease control.
I don't see any gameplay related reason as to why the game would not function equally well by moving the starting position to any other city of your choosing.
As an aside, note that Essen is a map location. It's a fairly minor German city of around 500.000 inhabitants, but included in many games because of its relation to The Spiel fair (the world's largest board game trade show)