The problem is you have to convince 2 things. First, convince people to pay for pornography when they can find most things online for free. Second, convince people to use their Bitcoin to pay for the porn. It's a big hassle I think and that's why it hasn't taken off. Or maybe demographics of porn payers are just not Bitcoin users?
Those two things are actually quite a bit easier than you might think. Many people pay for porn, especially when it comes to live cam websites like camsoda.com, chaturbate.com, and even bitcoinporncams.com...
Hassle? Isn't the whole point of Bitcoin that it should be easy to use and spend? If I were going to pay for porn I would much rather use my Bitcoin than a credit card. No need to worry about giving out personal details, or sketchy recurring charges.