I agree? Also, you categorized Fisheye Placebo as an American manga. Which it is not. And I don't think Undying Happiness is either but I can't say for sure. Alls I'm saying is, just because it doesn't have an anime doesn't mean it isn't big.
Also, marketing. Besides the fact that anime has its roots in Japan, it's just a much bigger deal over in Japan because of their target audiences and content than it is here in North America, which is ruled by action movies and superhero films (which it has been for a while now).
I'm just confused because you're talking about manga and anime on the one hand, and then webcomics on the other. If we're talking about anime, then it's no wonder it's more popular in Japan (besides, there aren't a lot of American-made animes that are actually good; just like how there aren't a lot of Canadian-made action and horror films that we can write home about either /sigh). If we're talking about webcomics, then yeah, there's a much larger demographic for webtoons and webcomics out in places like Korea, whereas you come to North America and, once again, people are more for the superhero comic books that you see from Marvel and DC because those are the things most of the generations that have been exposed to it have grown up with.
But webcomics are starting to grow in all corners of the world now, and that's just something that comes with time. It's all about marketing and catering to your target audiences.