That would be because less folks pay for porn, thus driving up the operating costs because of a lower consumer base.
This is why theft can destroy industries, because it simultaneously devalues a product while driving up the cost to the consumer. If I have to make $10,000 bucks and have a consumer base of 10000 people, everyone only needs to pay $1 buck for me to break even. I could charge $2 and double my investment, and expand. But if 5000 steal the product instead of paying for it, charging $2 bucks is now necessary to break even. When 90% steal, I now have to charge $10 bucks to break even.
I mean, everything is more expensive in general due to inflation (IE: government debasement of currency) but it's worse for porn because of that, even more than other entertainment industries because folks still won't talk about it the way they will other forms of media.