Because the characters for Japan's monarch is "Emperor." Japan's emperor is referred to as "天皇" which literally means "Heavenly Emperor." The word for king, "王," is derived from the Chinese word for king. As per its Chinese origin, a king (wang) is subservient to the emperor (huangdi), as per the Persian Pashishah (King of Kings) and the Western Basileus/Imperator/Emperor.
In the old days of Japan, during the Yamato period, Japan was divided into a variety of small chiefdoms and kingdoms. One of these eventually subjugated or allied with enough of the other ones to declare themselves the head of all the other kingdoms. When Chinese thought was directly imported into Japan, the Japanese adapted some of the conventions of China, most notably in a letter written from the Japanese Emperor to the Chinese one as a greeting from the emperor of the "rising sun" to the emperor of the "setting sun." Needless to say this did not go over well with the Chinese.
fyi, you'll find some previous discussion here