Answers:
because a single serving would be to small to get culture to buy. they can actually trademark more money buy selling you a 1 liter bottle of soda then they can the 12oz which is really the size we should be drinking. Besides it's close to eating at McDonald's. Most will super-size even though they are not that hungry and differently don't want the calories.
Other Answers:
To sell more product. I would own thought that was noticeable.
The size of the bottle actually *is* a single serving.
However, this is normally broken down into two servings, so that the nutritional information looks better (remember, it's now divided by 2!)
they label 8 oz. cans
So they can engineer more money.