My educated guess is that there is a cadre of local engineers whose market includes similar buildings and that the deterioration was ordinary enough that the engineers could draw on years of experience to say "it is not likely to collapse because other buildings like it have not collapsed."

As I have said elsewhere, the only thing I suspect is unique is that this was the first building to spectacularly fail.

It failed with people in it because there are tremendous disincentives toward saying the sky is falling even when the sky is falling. It's bad for business and it's bad politics.

Or to put it another way, suppose it was bad design or bad soil. Do you think that the same engineers, owners, contractors, and government agents did everything right on every other building? Think of how hard it is to keep salt off of rebar during construction next to the ocean. Now do so when there is every incentive to save money and no incentive to predict catastrophe.