The Irken Empire builds on perfection. The image it tries to force through, that is. The Irken society is superior, better, and must uphold this appearance to secure its position as it forces its rule onto other worlds. All other species are below the perfection that is the Irken soldier; even simple and short drones can elevate themselves through comparison to other non-Irkens. Tallers would prefer enslaved fellow Irkens as servants to the slaves from conquered worlds, who instead will be forced to work in factories and prisons. Why should they stoop down to deal with something so inferior as anything non-Irken?
The Irken is perfect and superior. And any signs of the opposite are doomed to be eliminated.
They are not even acknowledged as Irkens, but as monsters or mistakes. Freaks to be dealt with as swiftly as possible so they cannot blemish the perfect Irken society with their very existence. Cannot have something walk freely as the living proof of I