My understanding is that there had been an ongoing concern on /r/piracy that they would get shut down at some point, that this had been a concern in the past, and so the other stuff like the API restrictions and the rest of the spez drama was kind of just adding to the big factor pushing people away – that the community could vanish at any time.

The lead mod on /r/piracy also set up a dedicated instance – there was definite commitment – made it clear that he was making the move, and was demodded on /r/piracy, so there were factors creating more inertia.

Those are all factors that did not generally exist for other communities.