1) Sugar is addicting, and there is significantly more sugar in soda than other drinks [Google it]
2) Soda is very refreshing. This is important in warmer climates. If it is 98 degrees outside (37C), an orange Fanta does a much better job of making me feel better than just water
3) Soda is available everywhere and is the de-facto drink with most meals. Just like Italians drink wine with dinner, Americans have soda. This is perhaps the most important factor: If everyone is drinking soda, why wouldn't you?