My idea is that Requiem affects humans in certain ways that no other forerunner superstructure (Halo fortress worlds, Shield World Dyson Spheres, duh) does. If Didact can psychologically reshape any nearby sentient, we'll be seeing Del Rio's source of authoritative-abuse.
EDIT:
You know what? It wouldn't be surprising if Didact's source of anger besides his wife, is in part his anticipation to "manipulate" anyone whilst being suspended in a Timeless Xankara, achieving a near-god level of possession.
But to the why- wouldn't have the Didact used this back in the Forerunner-Flood and Human-Forerunner war? Likely not, that'd suck for plot progression.