Because writing code which contains a large number of checks and balances is generally orders of magnitude more expensive than human trust/judgment on the Ops team. Reading the postmortem makes me think that this sort of failure could have happened to anyone, and no-one really did anything wrong. The mistake was the blob store config flag not getting flipped, which is just a natural human error. The engineer who did the roll out could have been any of us. Given what he/she knew, he/she thought they had a good soak test (and a couple of weeks is a pretty good soak test) and made a call, similar calls he/she makes a number of times every day. This one didn't pan out.

I would hazard that most companies have a big red rollout button that is reserved for trusted engineers that will do a rollout without all the checks you're requesting.