It is usually a result of availability and upbringing. If a certain culture has rice readily available and another has corn, rice or corn will most likely become a staple of each culture's diet respectively. In addition, if someone grows up eating a lot of a particular food, they will be more likely to consume it as a staple food later in life. In terms of nutrition, many staple foods such as rice and bread are similar in their molecular construct, meaning they will be broken down into similar molecules for use in the body. For example, rice and bread are both broken down into essential carbohydrates, so it doesn't really matter which one you consume more (to a variable degree).