Because early Android devices had the back button on the right. See for instance the first Android device (https://en.wikipedia.org/wiki/HTC_Dream): the button order was according to Wikipedia "Call", "Home", "Menu", "Back", "End Call".

The standard Android button placement later changed to have the back button on the left, but Samsung kept the old order, probably so that users who upgrade their phone within the Samsung Galaxy series don't have to relearn their muscle memory.