You've got the chronology here backwards IIRC - Bulbapedia exists because Wikipedia got rid of "non-notable" Pokemon.
[1] https://bulbapedia.bulbagarden.net/wiki/Main_Page
[2] https://en.wikipedia.org/wiki/Wikipedia:Pok%C3%A9mon_test
Wikipedia doesn't _have_ to be just a encyclopedic overview of topics, it should have dive ins as deep as you want if there are people willing to write it.
That other set of editors could all just be Wikipedia editors, but then they'd have to play by Wikipedia's rules. They'd rather play by their own set of rules, and more importantly, have the ability to define their own rules. Autonomy. Sovereignty.
Now, in theory, there could be some "hierarchy of wikis" all maintained within one system, where different namespaces are maintained by different groups of editors (similar to e.g. Reddit with subreddit moderators) — but, because the goal would remain the creation of a single cohesively-presented encyclopedia, this would result in terrible inter-group conflicts about things that don't fall crisply into the magisterial domain of one group of editors or the other — e.g. rules for when a wiki page in one namespace, should link a topic of a wiki page in another namespace, and how that citation should be done.
(Imagine if editors in namespace A believed that a page in namespace B really should exist, and so kept linking to it, despite the editors of namespace B disagreeing; and the system hosting all of these constantly bubbling up the non-existent page to the attention of the editors in namespace B because it received new external links.)
The solution to this is decentralization. No hierarchy, no shared system, just reusable open-source software and federation through hypertext linkage entirely controlled by the origin. Which is exactly what you get when each wiki is its own website.
Fandom dot com is a commercial venture started by Jimmy Wales. Inferences are left as an exercise for the reader.
But does that actually describe reality?
Given how corrupt and petty Wikipedia's editors have become, the more complete and realistic reason might be that having a complex set of rules that allows some humans to pick and choose who makes it on Wikipedia gives people who would otherwise have little of it, some real world power.
And if you think humans aren't above basing their life activity over a petty bit of power, well, I've got some Reddit moderators to show you.
No, they don't, no more than Google Maps needs to "maintain" older versions of their imagery for access through Google Maps Timeline.
Curation should be directed towards informing the user and allowing them to make their own judgements regarding the content, not towards excluding content based on someone's completely-arbitrary opinion of "notability."
No matter how much hand-waving Wikipedia does on the subject, that's ultimately what notability comes down to: someone else's opinion.
It's also because people use having personal Wikipedia pages as a credentials boost, and they write puff pieces about themselves or their friends. If a person is notable, there will be multiple editors on their article, and the hope of the project is that multiple collaborators will reduce bias. If someone is not notable enough that people besides themselves and their friends would contribute to their page, there is room for substantially biased puff pieces. Most people take Wikipedia articles at face value, and don't delve into any of the sources cited, so that is a huge problem.
Wikipedia articles will always struggle with bias issues, but for the reasons you mention, there is no point in spending volunteer time verifying articles and removing bias when they're for people who aren't notable. That's why they just get removed.
https://en.wikipedia.org/wiki/Norinskaya
Or a random Kazakh football coach:
https://en.wikipedia.org/wiki/Andrei_Finonchenko
Or a library in Scotland that's planning to close:
https://en.wikipedia.org/wiki/Jedburgh_Library
Wikipedia has been capricious for years about what can stay and what must go.
By this I'm saying sure, you can have a dictionary with select words in it without problems... But when you label it as an OFFICIAL INFORMATION RESOURCE, it becomes subject to a higher level of scrutiny and objectivity that can't just hand pick what words are in it, there has to be a solid democratic aspect involved to managing the resource.
Democracy seems to be failing in many ways right now on public resources.
Think of it like code in an active open-source project. Someone needs to maintain the article: update it when house style changes, evaluate any new contributions to it as being valid or not, etc. Like code experiencing code-rot, a Wikipedia article rots if editors don't give it active attention.
This has exactly the implications you'd expect: it means that articles about things that don't change, are easier to keep around than are articles about things that might change; which are in turn easier to keep around than are articles about things that definitely will change.
Living people — where the article is basically living biography for them — are in that last category.
The "notability" requirement can be translated into editor-ese as a combination of 1. "how many people could we find who could contribute to this page", and 2. "how much demand is there for Wikipedia — rather than some other website — to do the work of keeping this."
Re: the first point about contribution, this is why Wikipedia doesn't let people be their own primary source — it's because, when that primary-source person eventually stops maintaining the page, who will then be able to take over the maintenance? If that's "nobody", then to prevent that, the page shouldn't be allowed in the first place.
Re: the second point about demand — the Pokemon Pikachu has its own Wikipedia page, because people expect Wikipedia specifically to have an article about Pikachu. Other Pokemon do not — because there's already Bulbapedia around to satisfy the demand for an encyclopedia with articles about Pokemon, and the pages from it are easily found in any search engine. If a different set of editors are willing to take on the maintenance burden for those articles in their own domain — and are doing a decent job of it — then why should Wikipedia's editors duplicate that effort?