The main reason why the Qin dynasty ended up like this is that the time of the Qin dynasty is too short. The Qin dynasty only lasted for more than twenty years, and then it died. This time, the Qin dynasty was not enough to make a large number of people in its territory agree with the Qin dynasty.
After the Qin dynasty destroyed the six countries, a unified Qin dynasty was established. However, the short ruling time made the Qin dynasty have no time to digest the resentment from the original land and people of the six countries. For example, Xiang Yu who destroyed Qin, Sean and three outstanding heroes of early han dynasty who assisted Liu Bang all had deep hatred for Qin Dou.
However, the time of the Qin Dynasty was too short, which led to many nobles of the six countries who had profound hatred for the State of Qin living in the Han Dynasty. It is customary for later dynasties to have dynasties before the Black Dynasty. There are so many dissatisfied people in the Qin Dynasty that the reputation of the Qin Dynasty will naturally not be good. After all, history was written by the victors, and the history of the Qin Dynasty was written by a group of people who had a deep hatred for the Qin Dynasty.
Another reason is that in the later generations, that is, after Emperor Wu of the Han Dynasty ousted a hundred schools of thought, Confucianism gradually gained the mainstream position in culture and took control of the right of public opinion. As we all know, the Qin Dynasty was a country ruled by legalists, and Qin Shihuang also did things like burning books and burying Confucianism. It can be said that the Qin Dynasty was very unfriendly to Confucianism.
Under such circumstances, when Confucianism controlled the right to speak, it is conceivable that Confucianism would not say anything nice for the Qin Dynasty. After all, the Qin Dynasty was a dynasty that burned books and buried Confucianism. It is wrong to put Confucianism and Confucian scholars together if the mainstream public opinion of Confucianism helped the Qin Dynasty to sing praises. So the black Qin of Confucianism became something similar to political correctness. The Black Qin Dynasty proved the correctness of Confucianism in some way. If the Qin Dynasty is right, then the only thing wrong is Confucianism.
It is precisely because of the short time of the Qin dynasty that many enemies of the Qin dynasty lived to the end of the Qin dynasty, and the continuous discrediting of Confucianism for thousands of years was also the key factor for the reputation of the Qin dynasty.