Like the last time I answered a question, I will say that this is something that I've studied in passing.

The main factor behind rice being a staple in East Asia compared to Wheat in Europe was definitely climate. China's south is warmer, wetter, and received extra rainfall every summer due to monsoons blowing in storms from the South China Sea. This wetter and warmer environment was suited to growing rice crops, and in some areas even allowed 2 crops to be planted a year. Multiple crops of Rice in a single year, combined with the fact that rice already yielded more calories in terms of space used to grow the crop, made it the better choice as you could simply grow more food.

In China's north however, where the geography is dominated by the Yellow river, the land is colder, flatter, and drier than the South. Monsoons also affected the north differently. Instead of bringing increased rainfall, they instead brought loess dust from the north and west towards the area drained by the Yellow river. This dust however, is fertile and is suited to growing crops like Wheat and millet. So foods in Northern china incorporate more grains than Southern China, which uses much more rice.

The South of China was more fertile than the north however, because it was able to produce more food. The north even eventually began to rely on the south more for food after the completion of the Grand Canal.

Ebrey, and Walthall. "Pre-Modern East Asia to 1800". 3rd ed. Wadsworth: Cengage Learning, 2014. Print.

Although I only mentioned Northern and Southern China above, I should also mention that multiple crops of Rice in a single year were even more common in Vietnam, with some areas supporting 3 crops a year.