After Yang Dingtian's death, why didn't Cheng Kun secretly take Gan Kun's secret book?

In the story of Eternal Dragon Slayer, Yang Dingtian died in the secret passage of Guangmingding. A few years later, Zhang Wuji came here to find Gankun and moved to another place. He successfully practiced his magical powers, and his martial arts led to by going up one flight of stairs, and later he became the leader of Zoroastrianism. Then the question is, why did Cheng Kun, Yang Dingtian's arch-enemy, not secretly take away Gan Kun's great removal secret book after Yang Dingtian's death, but let Zhang Wuji get it instead?

Actually, Cheng Kun really misses Yang Dingtian's secret skill of moving martial arts, because he thinks that if he has this peerless martial arts, he will be super powerful, and then he can successfully destroy Zoroastrianism, but things are not that simple. After Yang Dingtian's death, Cheng Kun had a chance to steal the secret script, but he gave it to Zhang Wuji in vain.

Tell me the reason and you will make a lot of jokes. In fact, it's not that you didn't steal it, but you went to the secret passage for a long time and didn't find it at all. He saw a sheepskin, but when he opened it, there was nothing in it, which didn't look like a martial arts secret book at all, but it was self-righteous and tried it with his own saliva, but in the end it didn't, so he walked out of the secret passage, very disappointed.

Many years later, Zhang Wuji and Xiao Zhao also saw the opened sheepskin beside Yang Dingtian's body. Zhang Wuji saw that there was nothing, so she was ready to throw it away, but Xiao Zhao didn't agree. She used her own blood to make the martial arts secrets on the sheepskin appear. It turned out that Xiao Zhao knew something about the relocation of Gankun. In this way, Zhang Wuji learned to move Gankun smoothly and became the leader of Zoroastrianism. It turns out that Cheng Kun didn't go at all, but he didn't find it at all.