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

Why can only the defiled manas give rise to the 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 ignorance as a condition and produces the volitional formations of the manas rooted in ignorance; taking the manas's ignorant volitional formations as a condition, it then produces the bodily, verbal, and mental actions of the six consciousnesses rooted in ignorance, 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 deliberative choices; after the manas makes its choice, the six consciousnesses arise to assist the manas in carrying out its activities. Therefore, if the manas is defiled and has ignorance, the six consciousnesses, acting as its assistants, will help the manas carry out defiled karmic actions. However, if the manas, though defiled, does not choose defiled actions, the mind consciousness can rely on its own purity to perfume 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 habituate oneself to wholesome dharmas and use these wholesome dharmas to perfume the manas, thereby breaking through 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.