Why can't Xuanzang be the fifth or sixth ancestor?

Because Xuanzang and Huineng have different opportunities. First, there are many Buddhist practices, and people with different opportunities practice in different ways. Master Huineng is a Zen master, and Master Xuanzang may be a monk of other practices. He may not have practiced Zen, so he can't be the fifth or sixth ancestor of Zen. Second, because of their different opportunities, their missions are doomed to be different. Just like in society. Xuanzang was destined to go to India to learn Buddhist scriptures and strengthen Sino-Indian exchanges, while Huineng was meant to spread Buddhism in China, including Jin Qiaojue who came to Jiuhua Mountain from North Korea at that time. Although they are both Buddhists with different missions, they are both great and noble monks in Buddhism. They are all saints worthy of future generations' admiration.