Porplemontage unilaterally wrote that policy in 2012 (the failed proposal was from 2009), after killing this attempt to merge the level articles (
MarioWiki:Proposals/Archive_31#Remove_level_articles_of_levels_that_have_names), since it's actually better for the wiki to have more pages for search traffic reasons, lower loading times, easier inter- and intra-article navigation, and whatnot. Before that, merges were all the rage and the ill-fated proposal was the epitome of the movement, but Porple's intervention and MW:NAP marked the end of that era of thinking and the start of "the more the merrier" pro-splitting approach that we still use today. He went over it with the admins, but I can't remember if there was a public thing about it, or if he just started splitting MK courses or whatever and people caught on and eventually followed suit. Anyway, the rollout was pretty slow-paced iirc, but eventually everything got caught up to speed with the policy - except the 3D missions/episodes, but that's an oversight, and rather than updating the page, I'd say it'd be better to make a collab to split them.
As for
why the missions should get separate pages besides "because we said so", well, same reasons as all the ret of the things in the list: namely, it's better for search traffic, better for navigation (no more section-links), and means less clutter of the world pages (so easier organization of the info both on and off the world articles, and lower loading times for every individual page). For instance, the galaxy pages are already pretty long thanks to all the planet entries - its actually to the point where ; are being used instead of subheaders for the "enemies" "galaxies visited" and "names" aspects of the mission entries (examples (
Toy_Time_Galaxy#Heavy-Metal_Mecha_Bowser)) just to keep the pages' TOCs manageable - this is a
bad thing, and it would be much better to split these missions into healthy, standalone articles with proper subheaders. SMS and SM64 aren't as bad, but they're still pretty cluttered with the foreignname templates alone - "one infobox per page" was the in-a-nutshell idea that Porplemontage told the admins when he made the policy, but I'd say it could easily be expanded to be "one foreignname per page" too (not counting things with multiple names and whatnot, anyway, but you get the point).