Growing complexity of the division of labor. The number of different position in modern social structures have increased enormously, and increasingly specific skills are needed by individuals to adequately fill them. We all do not have the skills and attributes needed to be an adequate diamond cutter, or professional pool player. Capitalist economic system. When the production and distribution of a society's goods and services are regulated by market forces, enormous pressure builds on employers to fill occupational positions on the basic of individual skills and not ascriptive attributes.
Capital magnifies the significance of small differences in skill.