Why can a defiled manas only give rise to defiled first six consciousnesses?

According to the twelve links of dependent origination, the cause of transmigrating through birth and death is that when the manas has ignorance (avidyā), it takes that ignorance as a condition and produces the manas's volitional formations rooted in ignorance; with the manas's ignorant volitional formations as a condition, the ignorant bodily, verbal, and mental actions of the six consciousnesses arise, and these bodily, verbal, and mental actions settle into the ālayavijñāna and become seeds. The volitional formations of the manas are precisely the manas's acts of decision-making; once the manas makes its choice, the six consciousnesses will arise to assist the manas in carrying out its actions. Therefore, if the manas is defiled and has ignorance, the six consciousnesses, as its assistants, will help the manas create defiled karmic actions. However, if the manas is defiled but does not choose defiled actions, the mind consciousness can rely on its own purity to permeate and influence the manas's mind; if the manas permits the pure actions of the mind consciousness, then pure karmic actions will be produced. Therefore, cultivation means to permeate oneself with wholesome dharmas, and through these wholesome dharmas to permeate and influence the manas, thereby breaking the manas's ignorance and transforming the manas. Then the mind becomes pure, and in the future there will be pure karmic virtues to be enjoyed.