With the disclaimer that I've never operated in this space, I would imagine that a significant barrier to the creation of new dating sites is that most people who care enough to try to build a good service are spooked by the liability involved (hosting boatloads of personal info, getting entangled legally when things go wrong, keeping creeps at bay, etc). At least from my standpoint as a layman it looks like an absolute minefield compared to a nice boring spreadsheet SaaS or something.