Because who the people are, who administer the prison system, and who are the objects of the prison system's rehabilitation program, matters. Humanity isn't a mass of identical copies. Certain traits, that might be highly conducive to effective rehabilitation, concentrate in certain regions of the world.

A smaller polity also tends to mean a much more accountable government which might be expected to be much more effective at implementing any policy.

>>Their GDP per capita is almost identical to the US.

Nominal per capita GDP is $5,000 higher.

The country also has a population that is 80X smaller. Smaller population size tends mean much smaller regional variations in socioeconomic conditions and fewer regions with the extreme conditions associated with high crime rates and pervasive criminal sub-cultures.