Because in Europe concentration camps are used to refer to Germany's extermination camps for Jews, Prisoners of War and children. So an alternate name needs to be found for a different thing.
“Concentration camps” existed before WWII, at least back to the Boer War. The US “internment camps” were concentration camps in the usual historical sense.
In the case of the German exterminatiom camps, “concentration camp” was a euphemism.
The extermination camps were usually next to concentration camps, where the survivors could live to work in the extermination camp. Some of the confusion is that the complexes are called "concentration camp" like Auschwitz which had 40 individual concentration and extermination camps. Probably because the concentration camps came first and exterimination camps were added on.