Because each country (and state) is very different from each other in a number of ways. It greatly simplifies the comparison when you say 75/100k vs 751/100k and that the only difference between those two is whether or not the justice system is seen as punishment or rehabilitation.
There are so many things that play into why those numbers are different that it is almost staggering: education, health care, cost of living, cultural/ethnic diversity, tax system, etc.
Norway has fewer people living in it than the county, I live in. In the county, we have overcrowded single prisons with more prisoners than the entire Norwegian prison population.
Add to this the struggles of getting rehabilitation to stick, when upon leaving prison a prisoner is likely to be picked up by his/her fellow criminals. Rehabilitation in prison fails when they leave prison and just return to their old "job." So, in order to get rehabilitation to stick, I need to do more than just implement it as the framework for my already overcrowded prisons (which have internal gangs that make rehabilitation challenging as well).
But, I get it. Norway is awesome and is able to have a prison system that rehabilitates criminals to be productive members of their society. The counties and states that make up the United States have plenty of challenges before them and how to reduce our criminal population is just one of them.