This is because China is an ancient civilization with a culture of 5,000 years and a long history. Every period has its products. Especially Chinese characters, each period has its representative products. In particular, the big seal and the small seal represent people in different periods respectively. Before the Qin dynasty, the writing was not unified. So all kinds of words coexist. This is the period when seal script prevailed, including Oracle Bone Inscriptions, Jinwen and Zhong Dingwen. Qin Shihuang unified writing, weights and measures, tracks and currency. In particular, it is a revolution to unify the characters and implement Xiao Zhuan. Seal script has become a common language in China after being determined by Prime Minister Li Si.
Therefore, because Da Zhuan and Xiao Zhuan are written in different dynasties, it is not allowed to mix them in calligraphy writing now.