Because that doesn't kill. And because that'd be a pointless thing to do since the Covenant can easily just glass the place.
A Marine in ODST describes it as a "flash". The slipspace rupture did manage to damage a lot of buildings, and managed to make the orbital elevator fall, but other than that, nobody was really killed besides the rest of the ODST platoon, who were still in mid-air, and dangerously close to the rupture itself.
[Edited on 07.29.2011 10:29 AM PDT]