Why is the calligraphy of Jin Dynasty "Shang Yun"?

Jin Dynasty was a dark political era. Literati had no hope for the society at that time, so they used calligraphy to express their feelings. There is a saying that their calligraphy is called "knowing to write first", which is interesting outside the law. So what they pursue is the meaning of calligraphy, not the neatness of fonts.