Several possibilities.
The Imperial penal system is an area yet to be mapped out in the SW universe, so it’s unclear where this prison intakes it’s inmates from. It’s possible the Empire segregates human convicts from other species.
It’s also possible that this prison only intakes inmates from certain regions of the galaxy, and this one being predominantly populated by humans.
Another possibility is that the Imperial government, being by nature capricious, tyrannical and xenophobic, simply subjects alien species to other, more cruel and vicious punishments e.g. enslavement, death by torture, worked to death in the mines, etc. with prison only being considered good enough for human convicts.