That's a very interesting and very difficult question you've asked. I imagine that your question focuses more on why different people have different features i.e. why skin color differs or why people have different facial features.

It has been argued that these features are due to the different climates in which people of a certain region have developed in. For example, people in the tropics are exposed to a lot more sunlight and so it is to their advantage to have darker skin to protect from ultraviolet. Or in a colder climate, people tend to have more body hair than people who live in a warmer climate. However, there are arguments to this and there are other factors that may play a role.

Scientists believe that the major driving force though, I believe, is evolution. At some point in time the environment that a certain group of people lived in provided a reason for those people to take on certain characteristics such as different eye color or skin.

Another driving force could be culture as well. It might be argued that Asians are shorter in general due to their diet of mostly fish and vegetables (this diet can be attributed to culture and to their environment as well!)

Your question is very complex and its answer has been argued over for a long time by many people (scientists, philosophers, anthropologists, etc.) I know that I definitely have not covered all of the bases here. The short answer to your question is that the environment and culture plays a role in why people of different ethnicity are different.