Because of troubles and ruling requirements, Qin unified the world after the Warring States Period. China in Qin and Han Dynasties was a unified country with a huge territory in the world at that time, and its population achieved a breakthrough because of peace. The old surname system can't keep up with the latest household registration scale, which is one of them. Eight surnames in ancient times were first used to commemorate ancestors, which implied China's ancestor worship and loyalty to clan. After the Qin and Han Dynasties, the country was unified, and the rulers demanded the people to worship and be loyal to the king. So he began to admire Confucianism, so loyalty to the monarch means patriotism, which means patriotism. At this time, the ancient surname that indicates birth becomes an obstacle, which is the second. Finally, I think it is quite important that the composition of rulers has changed. Surnames can be used to distinguish between high and low. At that time, it can be seen from the basic surname of the nobility. Your surname, you are basically human, and your surname is Zhao, so you can have a relationship with Qin Shihuang. However, all this has become irrelevant since the Han Dynasty. Why? Because Liu Bang is a farmer ... Yes, the prince would rather have seeds. Understand? The new ruling class is not an ancient aristocrat with eight surnames, but a pariah, so there are only two ways to make yourself taller. Renzuguizong, or start all over again. Both of them have used it to force themselves to be taller. So the surnames are combined into one.