A sugary soda is a great immediate post hard ride recovery fuel. Lots of sugar and (ideally) some caffeine. Try it sometime and the you will immediately understand!
Why Fanta? Fanta is pretty popular in Europe, not a niche brand like in the US, so its not as weird a choice as it might seem. Could be sponsorship too. Coke owns Fanta so there is the resources to pay to have it at the finish.