Insulin is a hormone, its job is to regulate the glucose level in the blood. If there's too much glucose in the blood insulin makes the cells absorb the glucose and store it.
One problem is that nerve cells do'nt talk with insulin - so if the other cells get the order to absorb the glucose (because we just had a nice glass of coke and the blood sugar went to the moon) they do it. But since there's no other glucose following the rush (like it would do with longer carbs like dark bread) the nerve cells don't get enough sugar (=bad, cause they need it for their work). Another problem: insulin decreases the decomposing of fat in the cells.
So, imagine we drank that glass of coke, blood sugar is high, insulin makes the surplus of sugar disappear. When the blood sugar get's too low there are other hormones and messenger substances that deal with that. But now we decide it's time for some cookies. Again a lot of sugar at once, insulin starts to be built and to work, blood sugar goes down fast, goes down too much - insulin production is stopped, other substances work to keep the blood sugar level as constant as possible.
With "longer" carbs that need more time to be broken into the smallest components this rise of the blood sugar content is slower, all the hormones etc. of the body can do the work they're supposed to do. The fast up/down with industrial sugar overwhelms the body. If the messages "more insulin" - "no more insulin" - "more insulin" etc. change fast the body can't start and stop the production in the amount of time needed or it might not have the material needed to build insulin. And if this loop doesn't work anymore you get problems like diabetes.