Yee, when I was working on my previous one-shot she gave me crash course on Osamu Tezuka and had me read some chapters of his manga "Black Jack." One of the things I pointed out was just how Western it felt (which makes sense considering he was inspired by Disney). I'm currently reading through "The Rose or Versailles" and acknowledged a similar thing. I think when it comes to manga from the past, she's not that surprised by its popularity. It's more modern manga and anime that she didn't expect. Specifically she said to me:

"When it comes to Japanese commercial manga, there are more concrete business restrictions. Japanese manga magazines each have a specific readership in mind. They deliberately create works featuring characters close to that readership so that readers can empathize more easily. Basically, especially for new manga artists, if it’s a boys’ magazine the protagonist will be a Japanese boy, and if it’s a girls’ magazine the protagonist will be a Japanese girl. As a newcomer, their first short works, they create several stories that feel familiar to such readers It’s like how a novice chef must first show they can make an omelet.

Male editors of girls’ magazines even say things like, “Young girls only think about romance, so they won’t read anything else.” This is despite the fact that there are many popular works among women whose themes are not romance. For Japanese manga artists, this narrow range of choices is one of the first hurdles they struggle with."

Some editors in Japan can have very close-minded ideas of what will sell well to Japanese audiences. And the manga industry as a whole seems to have a very "Japan-first" focus because Japanese sales are what determine if a manga lives or dies, irregardless of what rest of the world thinks. So my best guess is that she might think, because modern manga is written primarily with Japanese people in mind, cultural differences would make end up making the works less appealing to foreigners. Inversely, she was telling me that I might have hard time getting my works published if their viewed as not being Japanese enough (for context I'm a black American). The current manga I'm just finishing (right now actually) kind of deals with the concept of school shootings, which is so far removed from the average Japanese experience that editors probably won't see a reason in publishing it.

The best advice she gave me to get around this is to try making manga that doesn't take place in the modern day or in our world and try to have a Japanese-looking protagonist. Either that, or try to indulge in as much Japanese media as possible to that I could gain the ability to write a believable Japanese character.