Riot was beginning to rework its oldest champions like warwick, ryze, fiddlesticks starting in 2017. in 2018 they released the warwick rework along with a cinematic trailer and new lore. it was very very very good and left people wanting more Zaun content, but the game has a world with multiple regions each having a dozen or so characters and so it was understood that it would be a few years before they got back to Zaunite lore.
during this time they also make legends of runeterra which is a hearthstone copy that over the years became the biggest source of the setting's worldbuilding, especially outside of main characters.
in 2021 riot releases arcane s1, and a lot of side content in the game references Singed, Warwick, Zaun, all of this bad scary stuff that people had been wanting for almost 4 years. people got a glimpse of vander beginning to transform into warwick which had everyone really, really excited. It also made Singed terrifying because he was like nabbing people off the streets and doing horrific experiments similar to what he did to Vander, but in some cases worse. So one of the main things people wanted from a Zaun-centric series was showing the origin of Warwick, and more spooky stuff like the cinematic trailer. People were satisfied with how Arcane expanded on its main Piltover/Zaun characters, but Jinx and and Ekko aren't the original Zaun characters. Those would be Singed and Warwick.
Then Riot lays off half of its staff in 2022. It puts Legends of Runeterra on ice, but essentially confirms that it's on its way to being abandoned. Artists, writers, and everyone who had worked on the stuff people had loved and were anticipating more of were now gone. They stopped doing good reworks of old champions, got lazy with skins and started using AI art. The fun, lore and worldbuilding heavy side projects were no more. But who cares - all the lore is pretty much shoveled onto Arcane and its (presumed) spinoffs. This is when they decided to make Arcane canon.
People just wanted the horror itch to be scratched, and to feel excited about the worldbuilding for the game's existing characters. Then S2 makes Warwick unrecognizable in his transformed state, and lots of people didn't like the way they wrote his stuff in the show.