Why aren't there diet candy bars like there is for soda? Like Diet erfinger?

Because no matter how you look at it chocolate isn't diet food. The reason they can make sodas diet is because they substitute sugar for aspartame. Even if they were some how able to come up with a diet Snickers or Kit Kat it would probably taste like sh** since they would have to use a substitute for the chocolate - which we all know is what makes a candy bar so good in the first place.