It is said that manju stuffed with azuki bean filling was created because original manju was stuffed with meat (eating meat was prohibited in Japan while it was not originally prohibited by Buddhism).
しかし従来の饅頭は肉を使ってしまう（本来の仏教では肉食は禁じられていないが、日本では肉食を禁じた）ので、代わりに小豆を使った饅頭が考案されたと言われている。