Plessy was Constitutional then too, but that was overturned, as I believe Korematsu would be if it were challenged.
The big difference, however, is that it hasn't been. That it would be is also very far from certain. In fact, given the two cases' proximity, several of the same justices who decided Korematsu also decided Brown v. Board, overturning Plessy. Four of the six-member majority on Korematsu were still on the court in 1952. Brown was uninamous.