Well there must be some reason people who seem to have no problem spending multiple hours a day writing here about Musicbee refuse to update it and peope with questions or curiosity about Musicbee who could benefit from the information there refuse to refer to it. Why do you suppose those things are true?
It's funny, I also see complaints there about the inaccessibility of the forum. People are using it, there's just not a lot of overlap with forum users. Boroda and redwing did a lot of work on guides and plugins, before this site had its own section for skins and plugins. And endeavour1934 did the theme. But they've all dropped off the forum for the most part.
I think their WYSIWYG editor may be one tripping point. I can show anyone with an account how to disable it so you just get a text editor. Then you can do most formatting you might want to do with about 5 characters.
Added: another thing that crosses my mind is that responding to support requests on the forum is a very different animal from editing the wiki. On the forum you're responding to a particular situation, whereas the wiki is, as suggested, more of a manual. Ideally you're answering ALL the questions, though if course that's never 100% possible.
Also, on the forum you've got the back-and-forth of interacting with the user seeking support, which while sometimes frustrating, I suspect is also the most gratifying for us when it goes right. The wiki is not at all set up for that kind of conversation, which is why my response to anything more than the most basic questions is "ask on the forum" (to the point that I created a shortcut for linking to the forum in my responses). Relatively speaking, wiki editing is a solo endeavor and if you don't enjoy it for its own sake there's not much payoff.
