"Supply" is a key factor in sourcing livestock feeds.
Corn is the most widely used
chicken feed here in the U.S. Research has shown wheat to be a superior feed. However, wheat is also a better human feed than corn, so it is more costly due to supply/demand. The
chickens end up with the cheaper product.
In wheat growing regions, wheat is often the feed of choice for poultry, as the lower grades are cheaper, and more readily available than corn which must be trucked in.
Perhaps, in Korea, brown rice is not a food of choice for humans, therefore it becomes the logical
chicken feed.
I would imagine that corn would be a very high price commodity in Korea.
Price and availability will sway most people's decisions.