>They all contain similar content and almost none of it is taken down for illegality, at most because someone thought it was too ick and Ad money, or posted in a non-r18 area.
Simple, the internet is huge and some currently contended US law means that (past illegal content) a web host isn't responsible for content users upload. Copyright means that corporations can DMCA certain content off, but otherwise, there's not much to do. Companies don't WANT to have to look through every single post on a site that big, so if they can automate or simply ignore it, they will.
The legality in Canada is questionable, but Canada isn't looking through Reddit with a fine tooth comb (P.S. it technically is against reddit TOS to upload lol manga stuff. But it's hard to enforce on small subreddits). Canada may not even know what Pixiv is, and Patreon is often behind paywalls. It would just take a good (well, bad) mainstream awareness to answer your question, and the answer would turn out to mostly be "because politicians didn't know until CNN/Fox News blasted it".
That much was obvious during the U.S. controversy on Rapelay, a Japanese 3d eroge simulator that was not even sold in the US (nor ever has been), simply mislabeled by Amazon and visible in American's store for a while.
>Genshin Impact and Blue Archive are not popular because they are good games.
well we're getting very off topic but this is still an odd angle. There's no one reason why these games are popular and talking about fan art vs. game quality is arguing a chicken vs. the egg. Let's just agree that fan engagement in this day and age can be a force multiplier in terms of advertising something and spreading the brand. But 10,000 x 0 is still zero. Just ask how F-Zero is doing.