With a lot of free sites which I am not mentioning here but I am sure the majority here knows about, plus the torrents all over us where you can download anything you want for free, why should someone pay a premium (cash or bitcoin) just to watch porn. Free sites have everything now, its a waste of money.
Maybe some people loves to reward their hard works instead of downloading pirated movies from torrents for free which is illegal
Or they spend their money for something else such as live sex cam, go to night club or prostitute
People don't want to pay something if they can get exactly what they need for free/lower price.
True. That's why these days porn sites have made us to pay before accessing premium videos and are advancing in porn business
Not every videos are available on torrents
I am sure 99.99% are, we just need one person to buy the videos, and then he/she is free to share it with all of us if he uploads on a torrent site. Since torrent sites are illegal (not in all countries though) he can upload to file upload service and post on related forums the links. In the end everyone likes free stuff.