Earlier quoted context omitted.
It probably just never worked out that way. Usually everyone starts with documenting the distro-specific parts first, and then adds more and more, until even general parts are there. But at the same time, everyone probably thinks that those general parts are supposed in the specific projects' documentation, so nobody really cares about sharing. Until the point is reached that some wiki is so big and successful, that…
I think the sharing is easy. The maintaining is hard when there isn't clear ownership. How do the teams divide maintenance duties? How are vandalism and moderation dealt with across teams? How do disagreements between teams over style and quality dealt with? Cost of hosting split? All of these are possible to answer, but they are also much easier to deal with when you're not sharing between different organizations.
The hard part about sharing is the different syntax of wikis, which could be slightly different even in the same wiki-software. Then there is the organization-part, and the sync-process itself.
Of course, today, 25 years later, we do have better solutions and much more experience for those problems.
> The maintaining is hard when there isn't clear ownership. How do the teams divide maintenance duties? How are vandalism and moderation dealt with across teams?
I would think those are pretty simply, as they all follow the same rules. I mean, handling vandalism isn't much different between Arch or Debian, it's always the same. And moderation really depends on the chosen sharing-mechanism. Which brings up again the hard part, just on a different level.