Rice balls were first popular in Japan. Japan and China are both Asian countries, and they are very close to each other. There are many similarities in diet. Both countries live on rice. Many Japanese cultures belong to China. The influence of the Japanese brought the Tang Dynasty into the Tang Dynasty and took away a lot of knowledge and useful skills at that time. However, unlike China, the Japanese eat rice in the form of rice balls. Cook directly in the glutinous rice balls. This method is rare in China. Why is this method so popular in Japan? It's impossible in China.
There are actually many cultural factors! It can be traced back to a long history. At that time, chopsticks were not used in Japan. At that time, rice was rolled into a ball. You can eat it directly with your hands, and then the ball becomes a rectangle. This rice ball is easier to put on the table. It tastes easy. At that time, Japan was still a big rice country. This is rice. Without chopsticks, it would be natural to eat jiaozi. If you don't use jiaozi or chopsticks, don't use your hands.
Then there was an ancient war factor: the ancient people had to prepare their own dry food when fighting, while the Japanese soldiers at that time were carrying jiaozi on their backs. Because rice balls are simple and easy to transport, various ingredients can be added to add flavor, but they are easily broken. However, Japan is an island country with a small land area and short combat mileage, which is one of the reasons why Japanese rice balls have developed so far.