Why did Vespasian keep Domitian in a junior political position?

Vespasian kept his son Domitian in a junior political position during his reign primarily because of Domitian's behavior in the months leading up to his father's return to Rome. During this period, Domitian was described as being overbearing and irresponsible.1

Additionally, Domitian attempted to turn a military campaign in the Rhineland into a personal triumph, an act of excess for which he required his father's pardon upon Vespasian's arrival in Rome in 70 CE. Consequently, while Domitian held the title of princeps juventutis (imperial prince) and served as consul multiple times during his father's lifetime, Vespasian ensured he remained firmly in a subordinate role for the remainder of his reign.21