Because some people are perverts, mass advertising of their presence thanks to crime is equivalent of them egging each other on and the availibility of things for them to buy and indulge their sick minds makes some people a lot of money, illegal as it is.
Money makes the world go round. Children have been exploited for centuries, only now it's on the internet. If people cared that much about child exploitation why didn't they do something about the problems that existed even before the internet existed?