Why did the bureaucracy of the era of absolutism consist mainly of petty nobles and the urban bourgeoisie?

The petty nobility and the urban bourgeoisie were the main pillars of power. The government of the country gave these categories of the population a large number of privileges, for example, the nobles for their service received land with serfs, thereby increasing their well-being, improving living conditions. They were also exempted from taxes and duties. In return for this, the nobles supported the government, did not challenge any decrees, decisions made by the ruler.