But those two states account for roughly half of the total world population. According to your link, the US has 400,000 enslaved peoples, which is still less per capita, but the US is a lot more developed than India and China.

From the linked source:

North Korea has the world's highest rate of slavery, with about one in 10 people enslaved, followed by Eritrea (9.3%) Burundi (4%), Central African Republic (2.2%), Afghanistan (2.2%), Mauritania (2.1%), South Sudan (2%), Pakistan (1.7%), Cambodia (1.7%) and Iran (1.6%).

In retrospect, I should've said "violent conflict OR extreme poverty"