This is mainly because the Sima family established a dynasty, but they were very incompetent, and the invasion of foreign countries made the people of the Central Plains miserable and bullied at that time. Some things done by the Sima family were very contrary to human relations. That's why it has been criticized and its evaluation has been very low.

First of all, the Sima family directly took away the Cao Wei regime. It can be said that they were unfair from the beginning, when they represented the strength of the extended family. And at that time, no one could compete with the extended family. This also led to the fatuity of the extended family in the later period. The monarch of Sima's family even did something that violated human relations, which can be said to be the anger of people and gods.

It is important to carry out external invasions during their rule, when the Central Plains was devastated, but they were unable to stop these external invasions. At that time, the people of the Central Plains were slaughtered, although they finally fled to the south and established the Eastern Jin Dynasty. But the north was slaughtered by foreigners. It can be said that the Central Plains was devastated at that time. Bones.

So it is reasonable to say that the evaluation of Sima family has been very low in history, because they have not done what a monarch should do and have not established a strong and unified country. Although the dispute in the Three Kingdoms period ended, it quickly fell into the chaos of foreign invasion, which made the people in the Central Plains suffer greatly. Can be said to be a very dark period in the history of China. However, this dark history is caused by the incompetence of Sima family, so it is normal for future generations to have a low evaluation of Sima family.