Why is the hexagram six hexagrams instead of nine hexagrams?

In fact, since the appearance of gossip, there has been not only a divination in Zhouyi, but also the famous Lian and Gui Zang Yi. Shang's Textual Research on Ancient Gua in Zhouyi mentioned that there were many ways for the ancients to use gossip. Look at the examples of ancient hexagrams, or the recently unearthed fragments that are easy to hide. No matter which method or easy to hide, the eight classics are used in pairs. In this way, the original three stubborn became six stubborn. But why? Each of the eight diagrams represents a pure state, whether it is wind or thunder, until the image of everything later. Even if a classic six-pointed star can represent everything, it can only represent a constant state at the moment you use it. However, Zhouyi is a study of the relationship between people, people and nature, and people and things, and pays attention to the unity of opposites. Simple state (divination) is not enough to describe a relationship. So we should put the two states together (64 hexagrams) to describe a relationship. This paper extracts the principal contradiction (dynamic contradiction) from this relationship to study how this relationship will change in the world dimension.

?