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 its inmates from. It’s possible the Empire segregates human convicts from other species. That being said, in Rogue One, there is a scene of Jyn Erso lying on her bunk in an Imperial prison cellhouse with a non-human cellmate. This would indicate that inmate segregation is not standard policy within the Empire’s penal system. Confinement at hard labor seems to be a common punishment in the galaxy as both Imperial and New Republic jurisdictions use it. Prior to the Disney canon, the original Death Star was supposedly build by slave labor, mainly Wookiees, on the Imperial convict world of Desparyl.
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, medical experimentation, death by torture, worked to death in the mines, etc. with incarceration only being considered good enough for human convicts.