The power switches were in locked cages inside colos miles from the nearest Knight employee. The sysadmins could probably power off the boxes remotely, but the devs probably didn't have that access.
Yes, a "big red button" kill switch was the right answer, but they didn't have that, and it took time to work through several layers of corporate bureaucracy while losing $150,000 per second, time they didn't have.
Anyone with experience in the industry knows that, especially the day of/following a software release, "if in doubt, kill and roll back". Being out of the market for even a few hours is fairly easily to come back from, even in places where you are legally required to be in the market for a given percentage of the time in order to qualify for transaction tax breaks on your market making trades.
That's why on that day my manger called me after hours to come in early the next morning and test our "big red button" kill switch before the market open.