I'll give an example from the most recent week of manga sales:
https://myanimelist.net/news/52480193In the top 10 this week you had #8:
Hoozuki no Reitetsu, and it's hardly a series aimed at kids. Basically it interprets the underworld as a japanese corporation complete with all the drama and politics that entails, and plays on a lot of obscure traditional mythology. Young people with "pop sensibilities" would tend to be bored silly by all this.
And at #3 was
Kinou Nani Tabeta?Shirou Kakei, a straitlaced lawyer, cooks gourmet dinners for himself and his longtime gay lover, Kenji Yabuki, a carefree, hippie-ish hairdresser. The story is told through the lens of dinner preparation.
Basically nothing about this is designed to appeal to typical teen otaku sensibilities. And it outsold every harem thing in existence.
Meanwhile, #2 best-seller was
Saint Onii-san, a daily-life comedy about Buddha and Jesus sharing a small flat in Tokyo. It's observational humor about Japanese life and about their problems fitting in. Also not really something that's geared towards the typical otaku type.
So you have three seinen manga in the top 10 this week that deal with complex issues or taboo subjects. Plus, two of them got made into animes already. The main point there, is that when more "adult" animes do get made from top-selling mangas, the whole franchise is treated in the USA as some sort of obscure niche, and not "mainstream". Conversely, when some perverted fringe manga which hasn't even dented the "top 50" on the sales charts manages to get made into an anime for a niche audience, we often label that "mainstream anime" in the West.
EDIT: I'll give some examples of the difference in Japanese anime tastes to American, though I haven't looked into this as much as the manga
Here you have a
Japanese fan poll of the top animes they were looking forward to in Spring 2017, which is the most recent I could find. The top three were
#3 Natsume's Book of Friends (shoujo, sort of like a lighter and more emotional "Mushishi")
#2 Wararau Salesman (seinen, psychological drama)
#1 Attack on Titan
While Attack on Titan isn't a surprise, since it's season 2 of a popular series, the other two would be considered extremely "niche" in America.
However ... Natsume's Book of Friends is a best-selling manga that's been adapted to a long-running anime with
6 seasons. Yet - how many Western fans even
know about it? I like Natsume and have mentioned it before, yet western fans will only ever
grudgingly even give it a look, and consider it some "weird fringe thing" while Monster Musume, which doesn't even dent the manga sales charts in Japan, and got adapted into a
single season of anime, is literally a #1 chart-topping manga in the USA which is considered "mainstream anime" which tons of people watched or at least were talking about.
Here are the viewership numbers for the West, according to MAL scores.
https://myanimelist.net/anime/season/2017/springNatsume's Book of Friends, one of the most-anticipated series in Japan, was also one of the
lowest watched series by Western fans. Even though it's one of the highest-scored shows of the season (8.78), more Western people watched the boob-fest Sin: Nanatsu no Taizai, even though everyone
agreed that that was really, really awful (5.66). So, we just ignore the shows Japan considers good and watch the utter schlock instead, even while griping about how bad that shit is ...
Also note that
Uchouten Kazoku is supposed to be a brilliant show (I have it here gonna watch it soon), but Uchouten Kazoku S2 got around
half the viewers Natsume's Book of Friends did, despite also being one of the most highly-scored shows of the season, by those who
did watch it. So you have these awesome series which are popular in Japan, yet virtually
nobody in the West is actually watching them, instead we watch smut-fests galor, stuff that most of the Japanese fans don't even
touch, then we wonder why it all
sucks.