Why were some powerful Hebei clans excluded from high Tang offices?

During the reign of the Taizong emperor, the exclusion of the most powerful Hebei clans from high office was a strategic political maneuver intended to maintain imperial authority and balance power among the aristocracy.1

Taizong sought to prevent any single regional group from dominating the court. To achieve this, he actively balanced the influence of various aristocratic factions, including the Guanlong group from the northwest, the Daibei group from Shanxi, and the southern group from the Yangtze valley. By excluding the most powerful clans of Hebei—who considered themselves superior to the royal house—Taizong was able to curb their potential to challenge the dynasty's central control.12

Instead of relying on these dominant Hebei families, Taizong filled high administrative positions with members from other regional groups and the lesser northeastern aristocracy, ensuring that his officials were dependent on the emperor and the dynasty for their status rather than on their own birth and social standing.1