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

The reason for transmigrating through birth and death according to the twelve links of dependent origination is that when the manas has ignorance, with ignorance as the condition, it produces the volitional formations of the manas's ignorance; with the manas's ignorant volitional formations as the condition, the ignorant bodily, verbal, and mental actions of the six consciousnesses are produced, and these bodily, verbal, and mental actions fall away and are stored in the Ālayavijñāna as seeds. The volitional formations of the manas are the manas's choices and decisions; after the manas makes its choices, the six consciousnesses will arise to assist the manas in creating karma. Therefore, if the manas is defiled and has ignorance, the six consciousnesses, as its assistants, will help the manas create defiled karmic actions. If the manas, though defiled, does not choose defiled actions, the mind consciousness can rely on its own purity to perfume the mind of the manas; if the manas permits the pure actions of the mind consciousness, then pure karmic actions will arise. Therefore, cultivation means to habituate oneself to wholesome dharmas and use these wholesome dharmas to perfume the manas, thereby breaking through the ignorance of the manas and transforming the manas; then the mind becomes pure, and in the future there will be pure karmic virtues to enjoy.