Well, they have a higher Alexa ranking than the Pirate Bay, so it's probably easier for them to get advertisers. Also they're a near-monopoly. https://en.wikipedia.org/wiki/MindGeek On top of that, while porn may be objectionable to some advertisers' brands, it's still worlds better than the straight-up illegal content on the Pirate Bay.

There's a potential paying customer on a pornographic website, there's none on a illegal torrent tracker (or at least, there's less). There's always the possibility to advertise or sell live stream, access to other pornographic website, etc...

You never saw any pornographic ads on Pirate Bay? That's mostly the ads that I saw the last time I went there, but I wasn't searching for porn at all. I'm pretty sure though that people that goes to pornhub want porn, so theses ads should generate more profit on pornhub than on Pirate Bay.

We could also talk about brand association, etc... but I hope you already understood how it's easier to make money out of porn comparatively to illegal torrent tracker.