Wikipedia's notability rules are generally about ten/twenty years out of late, and seem to be designed on the assumption the internet doesn't exist. As you said, they often miss out notable works that aren't reviewed by what they class as 'mainstream' sources, and they also often end up failing to consider the amount of credibility someone may have in their own area of expertise.
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.
Subject-matter experts, such as distinguished professors, are considered reliable sources regardless of where their statements or writings are published. I'm not sure if it has always been this way, but it is currently written into Wikipedia's policy:
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.
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.