The reason is that carbs aren't the problem. Fructose is (largely) the problem. Refined grains (and refined sugars made from grains) shock the insulin system by causing a blood sugar spike (which can be especially bad for diabetics), but they don't cause long term effects. Fructose, on the other hand, which is found in ENORMOUS amounts in both HFCS, table sugar, and "fruit sweetened" things (and in small amounts in fresh fruit) causes kidney issues, must largely be metabolized into fat before being burned, puts a load on the liver, increases appetite and more. I've lost and kept off weight with no effort simply by learning some very basic biochem and switching in glucose, dextrose and maltose for fructose and sucrose.

For more informations (and scientific peer reviewed papers that back me up), I'd recommend googling for "fructose metabolism". There is also a great lecture about fructose on youtube called "Sugar: The Bitter Truth" http://www.youtube.com/watch?v=dBnniua6-oM