Natural Selection, or human adaptations through evolution.
The answer is "salience principle".
Salience principle, which expresses that we sort individuals based on what shows up at first noticeable and self-evident—that is, remarkable—about them. Skin shading is a remarkable trademark; it is one of the main things that we see about somebody. Since skin shading is so self-evident, it turns into a reason for stereotyping.
1 akhuvisarjaniyanam kantha
