well, not really sure... but it's crazy, I actually have some relatives that have a physical dependency. They asure they get headaches during the day if they don't drink soda.
particulary, where I live (Monterrey) we have a Coca-Cola subsidary, which means that they produce and bottle the sodas here, making them really accesible in terms of price and availability. And it's a really profitable industry that generate a lot of employment.
The marketing push it's crazy too, and both Coca-Cola and Pepsi always sponsor any big events, concerts, or sports teams. Also, a lot of restaurants offers "refill soda" with a really low price. Basically, it's cheaper to get unlimited soda instead of getting bottled water or something like that.
Oh, and in top of that, there's some stupid taboo that the "diet sodas" are disgusting. Many people don't like them because "they taste bad".
In general it's a very bad habit, and it's been like that since I remember. As I said, I hope that this tax cause that people reconsider their soda consumption.