The legal authorities treated those legal sites as if they were illegal, resulting in convictions of company owners and photographers who had been very careful to do everything legally.
Credit card companies stopped providing services to those legal businesses that remained.
At the same time, knowledge for how to safely access illegal material increased.
The supply of illegal material also increased as sexting became a common rite of passage for young people.
And at the same time, social media and the like created a large supply of photos of both famous and ordinary girls.
So whereas the legal authorities did everything they could to punish those who attempted to act lawfully, and made the penalties for acting lawfully as serious as the penalties for breaking the law, why would anyone bother to stay lawful?
And why would they pay when they could get everything they wanted - whether legal or illegal material - at no cost?
The question remains why these pictures ever became illegal in the first place, and why the authorities keep diligently pushing the idea even now. I believe it is simply that there is no way to rule innocent men. If one wants to stay in power, one must have something to hold over those one wishes to rule.