...when asbestos was banned, why didn't the builders go back to using concrete?Pure speculation, but perhaps concrete insulation couldn't be used on WTC because of the height of the buildings, i.e. the concrete would have added too much weight and destabilized the building.
Another tragedy was the Challenger shuttle crash. A NOAA veteran said that the cause was the removal of asbestos rinforcing and insulating fibers from the o-ring sealants. The new sealants had experienced marginal problems on earlier flights but not with ice handing on at launch time.