Firstly, even "free" porn makes money through ads. Lots of traffic x CPM always equals lots of money. Secondly, there is a huge business in high caliber and specialty porn. There are plenty of people out there with money to burn and a desire for something more specialized or higher quality than what you can find for free.
Edit: let me set up an exaggerated thought experiment for clarity. Imagine that the government commissioned a "one tru porn" project, creating a porn video that's a few hours long that is then hosted for free of charge for the world. It has all the normal sex acts anyone would want to witness, it has several very beautiful and very naked people in it, really, it's quite good. So why wouldn't everyone just watch that if they had a desire to watch porn? Quite simply put, because they want something else. They want a different experience, they want a different variety of people to look at, etc. The same phenomenon exists whether there is a small selection of free porn or a very large selection.
Edit: let me set up an exaggerated thought experiment for clarity. Imagine that the government commissioned a "one tru porn" project, creating a porn video that's a few hours long that is then hosted for free of charge for the world. It has all the normal sex acts anyone would want to witness, it has several very beautiful and very naked people in it, really, it's quite good. So why wouldn't everyone just watch that if they had a desire to watch porn? Quite simply put, because they want something else. They want a different experience, they want a different variety of people to look at, etc. The same phenomenon exists whether there is a small selection of free porn or a very large selection.