This is not a question that can be easily answered. It doesn't have a straightforward answer. Even the newest and arguably the most complete treatise in English on the topic of chopsticks explicitly states that no one really knows why the use of chopsticks took root in China. See page 5 of: A Cultural and Culinary History, by professor Q. Edward Wang. It’s published by Cambridge University Press. 2015. ISBN 9781107023963.

But it is clear that chopsticks originated in China, and eventually spread to what Wang calls the "chopsticks cultural sphere" which includes China, Taiwan, Korea, Japan, Vietnam, etc. The main reason for the adoption elsewhere was mostly cultural.

It is almost certain that chopsticks originated as cooking utensils for stirring food in a pot, for transferring food, etc. It was not the only cooking implement. There were other tools including spoons. In fact, spoons were the first primary eating utensils. In parts of China, porridge was common since early days. You would not eat porridge with chopsticks, but with spoons. The adoption of chopsticks as eating utensils came later. And for a long time, to this day, spoons are still used alongside chopsticks in most chopstick-using countries.