Let's see if we can put the context clues together.

All of the free ones were shut down. Only the paid one survived.

Well, I can only assume, but since fediverse services are hosted by real humans and not corporations, I'm going to go out on a limb and say they couldn't afford to stay afloat.

Another alternative is that requiring payment is a simple way to ensure that the people using your service are also 18+, since it requires a sign in form, acknowledgement, and then credit card. That helps when the feds come knocking on your door asking why you're letting minors into your service.