Why is the hexagram six hexagrams instead of nine hexagrams?

Because of evolution. Because of the rules, let's talk about evolution first. What needs to be clear is that Yi Gua is not a hexagram in the first place. At first, there was nothing but an idea of change, which represented chaos. Then there is a hexagram (one-dimensional) to represent yin and yang, heaven and earth. Yin and Yang, both heaven and earth. On the basis of the changes of Yin and Yang, we explored and found the changes of the four elephants (four seasons), and two hexagrams (two dimensions) appeared, representing the four elephants. With four images, this is the change of time, but it is not enough to find. Time and space are very separated. With one dimension (three dimensions), it becomes three hexagrams, symbolizing the world. The three hexagrams are complete, and time and space can be unified. Events are a set of ever-changing time and space. All right! Then put the three hexagrams together to become six hexagrams (four dimensions), indicating events. Of course, there can be more latitudes or more, such as nine. But why not use it? This is the problem of regulation.