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 this: because the manas has ignorance (avidyā), it takes ignorance as a condition and produces the manas's volitional formations rooted in ignorance; taking the manas's ignorant volitional formations as a condition, the ignorant bodily, verbal, and mental actions of the six consciousnesses are produced, and these bodily, verbal, and mental actions settle into the ālayavijñāna and become seeds. The volitional formations of the manas are the manas's acts of decision-making; once the manas makes its choice, the six consciousnesses 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, though defiled, does not choose defiled actions, the mind consciousness can rely on its own purity to permeate and influence the mind of the manas; if the manas permits the pure actions of the mind consciousness, then pure karmic actions will be produced. Therefore, cultivation means permeating oneself with wholesome dharmas and using these wholesome dharmas to permeate and influence the manas, thereby breaking the manas's ignorance and transforming the manas. When the mind becomes pure, there will be pure fruits of merit to be enjoyed in the future.