Largely because of the high sugar content. Soda pop is usually served at fast food restaurants that serve high fat foods. When you drink your soda, the sugar activates the pancreas. The pancreas produces insulin, an enzyme needed for digestion, but insulin also makes the body store fat. So, as your pancreas is going overboard from your soda, here comes the fatty fries and hamburger. Your body thinks it has to store that fat instead of burn it, because your body has more insulin than it needs for that meal. When at fast food "joints", try drinking some other beverage like lightly sweetened ice tea instead of a soft drink.

This may help you - the data from the McDonald's menu nutritional information put sugar in a new perspective for me when it comes to soft drinks. A small (16 oz.) Coke has 40 grams of sugar, a medium (21 oz.) Coke has 58 grams of sugar and a large (32 oz.) Coke has 86 grams of sugar. By comparison, a packet of sugar has 4 grams of sugar. Ha! Can you imagine dumping 21 packets of sugar in a quart of water and adding some flavoring and a fizz? That is a lot of packets of sugar if you were in a coffee house trying to mimic a Coke. People would think you were nuts and on drugs...ha! McDonald's has a Hot Fudge Sundae for dessert -- it has 48 grams of sugar. I guess a person could get two of those instead of a Large Coke and be "even"...ha! It is a funny world.

Regular, non diet carbonated drinks have a huge amount of sugar.  Typically 10-12 spoonfulls of sugar per drink.  While the diet drinks do not have that amount of sugar, they do have an artificial sweetener which increases the body's craving for sweets.

Softdrinks causes weight gain because it contains high amount of sugar which is metabolized to produce body energy. The excess sugar is stored in the body as fat.