Apples and oranges here, the US is not a developing nation. The single largest contributor to the decrease in global poverty you cite is a communist country.
A communist nation that realized that the communist way of state ownership and central planning doesn't work and implemented private ownership and a market-based, capitalistic approach to their economy.
Cuba would be an example otherwise? Despite having the nearby superpower trying their hardest to destroy Cuba, their communist system continues to work fine
Cuba is an abysmal failure. People have their occupations set by the government, depend on meager monthly state rations, and have almost no disposal income for consumer purchases.
That's why people flee Cuba for the US and not the other way around.
Cuba was in much better shape than Haiti on the eve of the Cuban revolution. It is only due to its natural advantages and its starting position in 1959 that it is still doing somewhat well relative to very low-income countries like Haiti, despite the damage done by communism and US sanctions.
>Although corruption was rife under Batista, Cuba did flourish economically during his regime. Wages rose significantly;[119] according to the International Labour Organization, the average industrial salary in Cuba was the world's eighth-highest in 1958, and the average agricultural wage was higher than in developed nations such as Denmark, West Germany, Belgium, and France.[119][120] Although a third of the population still lived in poverty, Cuba was one of the five most developed countries in Latin America by the end of the Batista era,[121] with 56% of the population living in cities.[122]
>In the 1950s, Cuba's gross domestic product (GDP) per capita was roughly equal to that of contemporary Italy, and significantly higher than that of countries such as Japan, although Cuba's GDP per capita was still only a sixth as large as that of the United States.[119][123] According to the United Nations at the time, "one feature of the Cuban social structure [was] a large middle class".[123] Labour rights were also favourable – an eight-hour day had been established in 1933, long before most other countries, and Cuban workers were entitled to a months's paid holiday, nine days' sick leave with pay, and six weeks' holiday before and after childbirth.[124]
>Cuba also had Latin America's highest per capita consumption rates of meat, vegetables, cereals, automobiles, telephones and radios during this period.[120][124][125]:186 Cuba had the fifth-highest number of televisions per capita in the world, and the world's eighth-highest number of radio stations (160). According to the United Nations, 58 different daily newspapers operated in Cuba during the late 1950s, more than any Latin American country save Brazil, Argentina and Mexico.[126] Havana was the world's fourth-most-expensive city at the time,[111] and had more cinemas than New York.[121] Cuba furthermore had the highest level of telephone penetration in Latin America, although many telephone users were still unconnected to switchboards.[122]
>Moreover, Cuba's health service was remarkably developed. By the late 1950s, it had one of the highest numbers of doctors per capita – more than in the United Kingdom at that time – and the third-lowest adult mortality rate in the world. According to the World Health Organization, the island had the lowest infant mortality rate in Latin America, and the 13th-lowest in the world – better than in contemporary France, Belgium, West Germany, Israel, Japan, Austria, Italy, Spain, and Portugal.[120][127][128] Additionally, Cuba's education spending in the 1950s was the highest in Latin America, relative to GDP.[120] Cuba had the fourth-highest literacy rate in the region, at almost 80% according to the United Nations – higher than that of Spain at the time.[126][127][128]
The association between quality-of-life metrics, like wages, and per capita GDP, doesn't disappear when an economy becomes developed.
And China is a highly market-based country, which saw a massive decline in poverty after its pro-market reforms that ended many state subsidies and recognized private property rights, as the Atlantic article I provided above explains.
