What is most likely the most important factor for this designation

the most important factor that is most likely for that designation is :

even though China's GDP is the third largest, China has the most population in the world. China's GDP/Person is still lower than average. Which is why China Still considered a Developing nation

Although China's total economic output, measured by Gross Domestic Product (GDP), is the third largest in the world, it is considered a developing nation.

Due to its enormous population of 1,300,000,000 people, its macroeconomic capacity is too great, by virtue of being the country with the largest number of producers of goods and services. But when the result of that production (the GDP) is divided by the number of inhabitants, we find a GDP per capita of $ 18,000, finding itself in the 74th position among all the countries of the world, very far from developed countries like the United Kingdom, with $ 45,000, or the United States, with $ 62,000.

This means that the Chinese government does not have as much money to meet the needs of its population, as more developed countries do.