Porn websites already know about the advantages of bitcoin. The advantages mean nothing if people aren't spending their bitcoins on the sites.
That is an important factor. At the moment, less than 100,000 people use Bitcoin actively. And out of that, IMO less than 1% might have subscribed to premium content, in sites such as Pornhub or YouPorn. This represents a minuscule fraction, when compared to the millions of premium users, which these sites are having.