Because not that many people outside of Japan are that interested in it. Similarly, there aren't that many people outside of India who are interested in Bollywood movies, so you don't see them played in cinemas except in places where there's a lot of Indian expats/immigrants. Art is highly cultural, so there's usually not that much of a market for it outside its culture of origin.

Anime/manga does do pretty well outside of Japan, compared to the cultural output of so many other nations (how many Czech movies have you watched lately?), but it'll as popular as you seem to wish it would be.