I would expect China limiting the movement of their rural populations from moving into cities might be a big factor.

Also it seems to end in 2025 before Iran started killing protesters in mass. Glancing around the index in question is very focused on civil liberties vs financial and life attainment in others.

- The detention of 1.8 million Uyghurs and other Muslim minorities, forced labor, and mass surveillance in Xinjiang. The destruction of Tibetan society and culture. The only comparable violation of human rights on this scale in the other countries is potentially Russia's war in Ukraine.

- China does not have competitive elections or an independent judiciary. The other countries do have these institutions to some extent, though deeply flawed and authoritarian.

- There is no freedom of religion in China or Iran. Russia persecutes some religious minorities, but tolerates different religions. Venezuela has constitutional protections for freedom of religion.

- There is no freedom of association in China. Independent trade unions, NGOs, and professional organizations are heavily suppressed and censored. These exist to a greater extent in the other countries.

- There is no freedom of speech in China. Political dissent is forbidden. All major media outlets are state-owned. Large parts of the internet are censored. Private conversations are monitored proactively. The other countries persecute speech, but in a less comprehensive, more retroactive way.