Webtoons are don’t online where it can be easily reached and translated while mangas are published as books that need to be hand drawn and edited as well as purchased in order to be translated..
Honestly Japanese mangas come out (or start out) in a weekly basis as well, but a lot of it is in print form for weekly magazines. Some artists that get more reputation can afford to slow down, and go on to a monthly or couple months gap. It can also depend on the magazine if it’s published seasonally (every few months).
For translators groups, it’s not cost effective at all to collect magazines, so most wait until the individual chapters are published into a book. Then someone needs to purchase and scan the pages of the book and translating can happen.
I don’t think there is a difference in the art method, but rather you realize that translators basically want to keep it ongoing and not go broke. You really only see speedy translations on highly, super popular manga (or online manga) because fans will actually buy the magazines and eagerly translate them sooner. It’s common to wait years for a print volume or series to be fully finished translated. And there are Korean manhwa books in black and white as well that are not webtoon/online based that go through the same process.
Also mangakas usually work alone, while manhwa artist have a whole team backing them up on the art.
That is wrong, however. Most mangaka does have assistant when the demand for their project goes higher or f they need to churn out works faster and better. Either for background, lineart or toning. They usually only got mentioned on the tankobon and afterwords. Some people owe prefer to do it alone or don't really have a good workmanship with their assistant (let me just mention HxH mangaka). Being an assistant of an established artist also good for them because it raised their level significantly because they learned directly under the established mangaka, and some people might even have a better skill than their employer when they started their own manga. If you look around you can find a lot of big /mainstream mangaka that I'd previously an assistant of some big mangaka.
*ps. Yana sensei have several assistant. They only got mentioned at the afterwords and behind the scenes on the tankobon like others.
There is only one chapter every month so it's only 12 chapters in a year and that is of course if nothing comes in between like the author taking breaks etc., I personally think for Black butler a monthly release is good since yana sensei always has such beautiful art in her manga and I wouldn't want her to rush that ヾ(❀╹◡╹)ﾉ~
This manga is 10 years old now.
Why do Japanese artists take so much longer? Korean Webtoons are so much faster.