Availability is obviously one big part as others have mentioned. Korea makes an effort to have the dramas produced in their country subtitled - especially in English which opens up the doors for translators to translate from English to their native language.
But I think stuff like Japanese dramas have bigger issues. They're more of an acquired taste.
I take a look back and I've watched four Japanese dramas this year and have enjoyed them. But they were all set in modern times and the acting and pacing could best be described as slapstick. And that's behind the times for most global audiences no matter where you go. And the people wanting slapstick can easily turn to anime where it's easier to portray than on real actors.
Now what do I mean by slapstick? I mean the main story, the ridiculous hook of the storyline is played up and exaggerated throughout the show. Usually it's a quirky feature of one of the leads like Takane no Hana-san where she constantly berates the male lead and it's called 'tsundere'.
It doesn't feel natural. It feels overplayed by the characters involved. And I do believe this is a common 'problem' for Japanese dramas in terms of attracting the global market (especially the Asian market). They overplay a character trait or a theme in the show and beat it like a dead horse. Did the story need to reinforce it that much? I don't think so. It wears thin after a while. Kind of a "haha she has feelings for the male lead but insults him publicly all the time".
And keep in mind I gave Takane no Hana-san a 7.5/10. I don't think slapstick is horrible but I don't think I could watch another drama like that for a few months at least which is why Japanese dramas won't ever rival say Korean or Chinese dramas on my watched list.
As for Chinese dramas being less popular - again, less availability to global audiences, censorship by the authoritarian regime and dubbing are all reasons it's not quite as popular. I really think it matters that a mouth lines up with the sounds being made and that's not as true from Chinese dramas (and I have probably watched more Chinese dramas than Korean ones at this point). What make the dubbing even more frustrating is sometimes it's spliced into the show very poorly. I'm not sure if this is because of re-writes or government censorship but dramas in 2021 will have someone talking and then their voice changes, same voice actor but obviously in a studio so the sound doesn't match the room or atmosphere. A real immersion killer.
This also brings up something interesting - Korean dramas do actually dominate in Asia and do really well in places like China. But remember Chinese dramas themselves tend to have subtitles so people with different dialects can understand what is going on. That might make many Asian audiences more accepting of subtitles where a German audience or an English audience would not be and why a Korean show will not be the hottest new thing in Germany or England or wherever but could be the hottest thing in China where subtitles for viewing is much more common and normal.