For starters, because (well some of it) isn't processed. Nothing added, nothing taken out. Sugar is still in the end made from a grain, and because it is heavy in carbohydrates is restricts insulin production, and since carbs are easier to process than insulin, your body is going to go for the path of least resistance. A little sugar here and there won't really do much to you, but constant consumption of sugar slows down insulin production and as a result of you taking in more carbs than your body is able to burn, the end result is obesity. If you're highly active, your body will burn the carbs, but you will still lack the insulin. Thats the reason most runners have a very short lived career, seeing as how lack of insulin and overloading carbs leads to weakening the immune system and doesn't promote healthy muscle growth. Thats where Proteins and Omega3's come in.
As for HFCS, it's so goddamn genetically processed by the time it reaches the consumer, it really is nothing but pure genetically processed carbs.