For instance, in the gaming world, I often look at Serebii.net as a good example of where Wikipedia's credibility priorities are misplaced. To them, it being a fan site run pseudononymously makes it less credible/reputable than a publication like IGN or Vice or what not, but in the actual community/among those who know of the topic, it's actually the far more credible source. The owner has written in magazines, been quoted by popular media sources, used as a reference by those seen as more reputable, etc.
But Wikipedia doesn't think like this. To them, credibility equals being paid as part of a major publication, and it doesn't matter even if you're a distinguished professor writing on your blog or academic site otherwise. And this then hurts the niche topics you mention even more.
https://en.wikipedia.org/wiki/Wikipedia:Verifiability#Self-p...
You're right about fan sites. It's harder for people outside of the fan community to evaluate whether a fan site is reliable, and that contributes to the skepticism you see.