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

Because the first six consciousnesses arise in dependence upon the manas's decision-making (the volitional mental factor, cetanā), and they arise for the sake of what the manas needs—whatever the manas needs, the six consciousnesses are born to accomplish it. Therefore, if the manas is defiled, the six consciousnesses will accordingly engage in defiled karmic actions; if the manas is pure, the six consciousnesses will accordingly engage in pure karmic actions. According to the fundamental cause of sentient beings' transmigration through birth and death as revealed by the twelve links of dependent origination, it is because the manas has ignorance (avidyā) that, with ignorance as the condition, the manas's volitional formations rooted in ignorance arise; and with the manas's ignorant volitional formations as the condition, the six consciousnesses' ignorant bodily, verbal, and mental actions arise. These bodily, verbal, and mental actions all settle into the ālayavijñāna and become seeds.

The manas's volitional formations are precisely the manas's decision-making actions. Once the manas makes a decision, the six consciousnesses are born to assist the manas in carrying out its actions. Thus, if the manas is defiled and has ignorance, the six consciousnesses, as its assistants, will help the manas engage in defiled karmic actions. However, if the manas, though defiled, does not make a decision toward defiled actions, the mind consciousness can rely on its own purity to perfume the manas. If the manas assents to the mind consciousness's pure actions, then pure karmic actions will be produced. Therefore, cultivation means to perfume oneself with wholesome dharmas, and through these wholesome dharmas to perfume and transform the manas, thereby breaking the manas's ignorance and changing the manas. Then the mind becomes pure, and in the future there will be pure karmic virtues to be enjoyed.