Another way to ask this question is: why is go less popular outside east asia? It's huge in Japan, China and Korea, and although it has its fans in Europe and the US, it's much more niche there.
I've got to admit, since I got kids I moved back from go to chess. Somehow it's more accessible, because everybody here knows chess.
I'm not even convinced Go is all that popular in Japan compared to Shogi or Mahjong. Even Gomoku (five in a row, no capture rule) seems more popular. I can't really say anything about the popularity of go salons since I have yet to visit one but I can't help but notice that the game is never included as a minigame in Japanese titles like the Yakuza series (which has all sorts of other board minigames) or even Clubhouse Games for the Switch (51 games, one of them Gomoku, but no Go). It'd probably take another Hikaru no Go to increase popularity again...
I suspect the lack of Go in Yakuza or Clubhouse has more to do with the computational complexity of providing a decent computer opponent to play against.
There are multiple factors and I’m not sure what the exact cause is either. The relative difficulty to build a playable bot is one. A Go game being relatively long and taxing may be another. Go also traditionally has a reputation of being a game for bureaucrats and intellectuals, while Shogi being a more people’s game. The obvious parallel between the battlefield also makes Shogi more popular among samurais, who yakuzas draw a lot of inspiration from.
In general I think the two are of comparable popularity in Japan, but I wouldn’t be surprised if real yakuza people do like and play Shogi much more than Go.
Decent relative to what? These games include various card games too which in theory are even harder for AIs to beat pros at. I'd be surprised if the other minigame engines are actually decent to a skilled amateur on their hardest difficulty. The AI complexity is also irrelevant to many of these games being playable with a friend in person or online.
The length of the game is also something easily tweaked, just default the board size to 9x9 and offer 13x13 and 19x19.
What I really liked about the anime, assume it's the case in the manga too, was the sheer number and diversity of regular, older people.
Obviously they weren't trying to paper over that it was mainly an old person's game in Japan at the time (I would assume it still is, too). But that didn't make it seem less appealing.
Also, the portrayal of foreigners in that series is worth a study in itself.
Tradition + first movers advantage/network effect. We have a centuries old tradition of playing chess in europe, we dont have a tradition of playing go (and vice versa for east asia). And as you said, once everyone already knows how to play chess it's much easier to also play it vs a relatively unknown game (part of the reason why D&D is the most popular tabletop RPG - everybody knows D&D!)
I've got to admit, since I got kids I moved back from go to chess. Somehow it's more accessible, because everybody here knows chess.