Trust. You have to be sure the people you're working with are diligent, careful, and won't turn you in (and likewise for them).
For example, you could operate (your instance of) your pseudo-legal service yourself, but open-source the codebase (sticking it on GitHub even) and accept patches from anyone.
You could also implement a simple replication architecture for your service, where at any time one node (yours) is considered to be the 'canonical master', and then other nodes can join the network as slaves, receive replicated state, and run as mirrors. Sort of like Linux package repository mirrors.
With such a setup, arresting the original maintainer just means 1. a codebase fork by a new maintainer, and 2. a (probably manual) network master-node election.
And that's just a setup that lets you stay in complete control without having to trust anybody (since you still "own" the codebase, and the master node, until you disappear.) If you are willing to relinquish control to the system itself, you could just build your service on a DHT or a signed store-and-forward hierarchy or a blockchain or whatever. There's no reason that things that are essentially "a BitTorrent tracker exposed through a website" need to have their canonical state anywhere at all.
What.cd open sourced Gazelle, the source code for the website and tracker. The core development team accepts pull requests, but they are themselves insulated from the owner and operator of what.cd.
Since opening Gazelle, many other private trackers have adopted it.
[0] https://ipfs.io/
It wouldn't surprise me one bit if ethereum came to be a way to host illegal websites considering bitcoins main market is for illegal transactions.
If someone starts cooperating with some type of law enforcement you're screwed, and if anybody gets caught and is facing a 50 year charge unless they cooperate, you can probably bet on which one they'll choose.
I mean, I would assume it's because it's shady business. Even if you made it multi-national, there are probably only a few places where you could reasonably keep the server hidden for an extended period of time (Keep in mind, these aren't Tor services or anything, they're pretty out in the open as far as I can tell) - and you're making money off of ads, so actually getting paid is another complicated issue. And really, if you only need one person, why bring in the risks of a second one? I don't think you'd be very concerned about "keeping the scene alive" if you're going to jail.
Wikipedia is easy in comparison because most countries don't throw you in jail for running a Wikipedia server.
To answer your question I think these are one man shops because they get shut down before reaching the size that you could have a larger team involved. Maybe if someone without a profit motive started something up they'd build a system that solved the problem above and could therefore last longer than a lone individual's contribution.
You don't have to shut it down, you can simply prevent access to it, like in China. Problem solved, if that's something you considered a problem in the first place.
Or you could just download the Wikipedia CD. It contains 5500 articles from 2007:
https://en.wikipedia.org/wiki/Wikipedia:Wikipedia_CD_Selecti...
Neither of those two are very up to date of course, but it shows that it can be done.
USA government always have a way though, there's no way either place would stand up for their own citizens once the USA start threatening sanctions.
This seems like a bus-factor problem. Why does it keep happening? Why aren't these sites being run by multi-national teams that can survive a loss like this?
Even The Pirate Bay is "just" Swedish, so a sufficiently-motivated Swedish Government could shut TPB down. Meanwhile, there's no single country that could shut down e.g. Wikipedia.