> Norway is also 85% ethnic Norwegians (making for a pretty culturally homogenous society).
Can you cite a source that shows that this matters?
It's pretty obvious how the difference in methods between the two prison systems can easily explain the difference in outcome, without having to jump to the 'race/culture'-card.
Simply put, having prisoners cost money. You have to pay for housing them, and the opportunity costs of them not being able to contribute to the economy is probably many times bigger.
European systems recognize this fact, and try their best to get prisoners out of prison as fast as possible.
Prisoners in European systems can get counseling for mental problems, access to education, and the structure in the prisons allow the prisoners to adopt a 'normal' lifestyle. They are supposed to get up themselves in the morning, go to their education, or labor, and having time off in the evening to do their own thing.
Although American media often shows otherwise, most crimes can be explained by circumstance, rather than a particular 'evilness' of the perpetrator. Nobody wants to be a criminal. European systems help prisoners to live a life free of crime. American prisons make it harder.