Why was the shogunate called the king of Japan by Ming Chengzu?

Because the Ming dynasty didn't know much about Japan, and it was not in the mood to study so much for a small country. Traditionally, the rulers of all countries are kings. Therefore, no matter whether the actual ruler of Japan is Guan Bai, Taige (Toyotomi Hideyoshi) or the shogun, or the emperor himself, they are all called Japanese kings.

During the Wanli period, when Japan fought in Korea, the two countries negotiated and promised to make Toyotomi Hideyoshi king of Japan. Toyotomi Hideyoshi is Guan Bai (equivalent to the Prime Minister), not a shogun. Later, the actual ruler of Japan set out from Tokugawa Ieyasu and became a shogunate general, but for the Ming Dynasty, no matter what generals and ministers were, as long as they were the actual rulers, they were still regarded as the king of Japan.