Why do the Compilers wear robes? Because the Pfhor made them.
One has to account for the fact that the Compilers are the product of a thousand years of Pfhor enslavement, probably stripped of most racial identity and made into something more... palatable to the Pfhor. There is one pic of native S'pht being captured by Pfhor, somewhere in the M2 terminal pics. They look like squishy brain-creatures that ride atop the little grav units you see inside the Compiler robe. Add a helmet and robe, and you're... here.
Remember that the Enforcers wear robes, too. Perhaps the Compilers are robed to designate them as directly subordinate to the Willful-class Enforcers.
