I have 2 thoughts which are probably just rambling and might not add much to the discussion:

1) Groups/websites/imageboards are ultimately defined by their do's and don'ts. If there's only one place that doesn't ban "X", then everybody who love X will come and stay to that place, as long as X is still allowed. Even if the place is welcoming for a lot more topics/subjects than just "X", it will ultimately dominated by "X"-lovers. That's why there's so much illegal content on the darkweb. It's not that the darkweb was created specifically for illegal stuff, but it's the only place that will host it freely.
Why would you post legal porn on the darkweb, when you could do it in the clearweb?
The point is: whatever rules heyuri allows, which other imageboards don't allow, will become it's defining trait.

2) The eternal question of all internet communities is: how to be in a group where people like things you like, and don't share things you don't like. That's pretty much the "culture". We want people who like the culture fit in the culture, and people who aren't interested in that culture to not interact. It's difficult to establish and enforce a culture...