Character development. Sho is pretty reactive but doesn't show very much agency. We also don't know much about any hobbies or his history or relationships. We get a little bit, but not much. The enemies are pretty cut and dry too. Obviously it has something that we enjoy, and it has a fairly large cult following, but it's not mainstream because of that lack of character.
edit: oh but of course it does improve later, but it takes a long time to get to the better character bits.