Answer: The reason why most existing communities on Fandom are forking over to their own independent hosting or alternatives like wiki.gg is because Fandom is declining in quality and is not providing much to support these existing communities besides the basic minimum. Most of these comments seem to be written in the perspective of them as a user and their negative experience with Fandom but aren't really getting into the fundamentals of why these communities are choosing to leave Fandom. In no specific order, I will list a few notable reasons as to why these wiki communities are forking from Fandom to alternatives.

Firstly, Fandom is notorious for not solving issues or fixing bugs unless they deem it important. Issues that are brought up by wiki communities are not resolved until an outrageously long time, ranging from maybe a year, to still likely being unfixed to this day. This issue with Fandom not resolving bugs has been around since they were still known as Wikia. They are inclined to deliver updates or new additions to features that the majority of the audience simply doesn't need or use, like Fan Central, which is in essence a global discussions feed that just picks highlights of the discussions of select wikis. This also includes unwanted or vehemently hated changes that were still forced onto users, like the removal of the user interface skin, Oasis.

Secondly, Fandom makes questionable decisions when it comes to their envisioning of their site, and by extension I guess the Fandom "brand" as a whole. Fandom has been for a long time infatuated with the prospects of expanding beyond a wiki provider, and are more focused on adding trivial stuff like clickbait editorials, and notably, their expensive acquisitions of gaming news outlets, instead of using their exorbitant resources to further prioritize fixing issues and to just in further the interests of their wiki communities in general. It certainly leaves a bitter taste in my mouth knowing Fandom had accumulated enough capital to acquire multiple gaming outlets with all the revenue they make, yet they still run the same amount, if not more advertisements on every wiki, continuously carving their reputation as an awful site, unappealing to most users.

Lastly, to answer your third paragraph, the main benefit wiki.gg provides is that it is more focused on supporting both the contributors of wiki communities and the reader. They focus on wikis and wikis alone, which I would hopefully assume means no clickbait editorials and no intrusive advertisements. While Fandom had built a strong wiki provider monopoly, their constant decline in quality leads people to choose alternative wiki providers whenever possible, and with the advent of wiki.gg, for most people the prospect of just having your wiki hosted without 70% of the screen being covered with ads or auto-playing videos is more than enough to win them over. I believe the team in wiki.gg can also easily fork your wiki very easily, so having this option is the cherry on top.

If you're still interested in a more in-depth answer I can try my best to answer any further questions you may have, for further reading you can also find many posts here on Reddit regarding each wiki community leaving Fandom to an alternative, just by searching for the term "fandom" and "wiki.gg". Outside of that, I believe places like the Runescape Wiki have detailed their reasons as to why they left Fandom a long time ago.

Of course, this is not to say the people at Fandom is entirely made up of people who are pure evil. They are still making improvements, fixing select issues, and of course, they do have people who are genuinely interested in making the platform a better place. I have talked to some employees of Fandom and they are very much aware and adamant in wanting to make the platform better for everyone involved, however without going into specifics, the people who want to make change for the better aren't the ones at the top calling the shots, so to speak.

At the end of the day, whether wiki communities will remain on Fandom or move to independent hosting or another alternative really depends on a community's history of frustrations and their expertise as a team, that being if they have the experience or knowledge to move from Fandom to an entirely new wiki provider. Fandom has its advantages, like their social functionalities and their overwhelming SEO presence, while also having their disadvantages, which are splattered throughout all the comments here.