I think their biggest problem is lacking a free trial
They have a 3 day free trial by default. And members can also give out a limited number of "guest passes" which act as a 1 week free trial.
I suspect their closed model is because it’s very expensive to host that data, and they want to make sure that whoever they put up there is worth spending the money on
That's probably part of it. It's also a marketing tactic. They're positioning themselves as a premium service. They want customers to know that if something is on Nebula it's going to be good. Similar to the way Apple positions themselves as premium by not selling a $200 smartphone, or Mercedez-Benz, or Louis-Vuitton.
It's also about trust. One of the things they're trying to do with Nebula is to provide creators a space to safely discuss controversial topics without censorship. But with that, along with the fact that they have a coop-type structure, comes the need to be able to trust that the people uploading on their platform aren't gonna be Nazis.