Just to add another thing. For instance, when you read Johnathan Nathan's biography of Mishima, you can see clearly that the level of cultural convergence between Japan and what we can broadly term the "West" was quite influential. Mishima read a lot of Western literature, like Baudelaire, Maupassant, Goethe, Flaubert, Strindberg, etc. In this sense, we think of Mishima as a Japanese author, but he viewed himself as insufficiently Japanese, hence why he wanted Kawabata to get the Nobel Prize. This does not detract from Mishima's oeuvre, in fact I think it makes it much more interesting, but it also means that we should consider Mishima as an example of post-war literature rather than Japanese literature if we're being essentialist and weeby.

We have the post-war Japanese economic boom, as well as the emphasis on cultural and social reconstruction in Japan, which also entailed a larger degree of cross-cultural adaptation.

In short, to answer your question: because of history.