i know i'm really late into this discussion and just made account only to reply here lol
wjsn in their debut era had a really long promotion, 3 months and keep doing 2 months promotion until happy era. and they did a lot of fansign to influx their sales, i think in their debut + 2 comebacks they already did 50+fansign. of course the fans keep growing especially due to yeonjung, their fans engagement, and having distinct concept. i think i wish were still their most downloaded song (before gaon erasing download chart).
and wjsn have niche fans in china probably due to yuehua name, still not as famous as bts or blackpink where chinese public know them, these fans also the one that brought half of their happy moment sales where happy (starship attempt to get public recognition) tanked hard both in public and fans even lots of k-ujung left fandom. And while wjsn doesn't have english fansubbers, their chinese sub content are everywhere in weibo made by c-ujung that lure more chinese fans into their fandom.
And of course while chinity activities in china gain more people to know about wjsn, not all c-ujung are chinity solo stans, in fact lots of chinity solo stans hate wjsn due to meiqi xuanyi were underpromoted there. but their appearances really help c-ujung bigger. wjsn probably have chance 1st win in dreams come true but mmld boom boom were blocking digitals in the show, and they were against mamamoo and sunggyu on the next week, they also have potential 2nd win in save me save you but blocked by soyou bcs starship give her comeback 1 week after wjsn. so they were bound to get win in boogie up thanks to good timing.
and ifans kinda underestimate wjsn popularity in korea, yea it's true they haven't got hit song yet but they still can enter top 200 gaon digital this year which only 14 gg can get that, even the last two comeback they have better digital than lovelyz, their fancafe often viral in korea due to their witty answers, mentioned as 1 of 4 funniest group from non fans, even the members also guest in lots of variety from popular ones to unknown one, and attend lots of events and festival too there.
tl:dr it just bcs wjsn have good engagement with fans and having lots of content that make their fandom rapidly growing both in korea and china.