They did a blog post on it that I can't find right now. Essentially, they made it quite a while ago (2002? I don't recall exactly), and all other options were too slow. Their backend apparently has to do a lot of work calculating match scores when users search.

Found a post by one of the developers describing the decision in some detail: http://www.brightjourney.com/q/okcupid-write-web-server

They've also open sourced their server: https://github.com/okws/okws

They were in much the same boat as I was back then: whatever you could install out of the box was simply too slow. Inefficiency would have caused us to die on hosting costs so we had to build our own webserver and so on. It's gotten a lot easier over the last decade and a half to build a performant website.

If Ok-Cupid were started today I highly doubt they would do that the same way. But given when they did start it I figure they literally did not have a choice.