Perhaps a room-level fault was considered too remote.
Or the level of backup was too costly.
Several times per year you have disruptions of similar scale from severe weather events. At some point it becomes cheaper to just shut down for a day, or so, then it does to have a sufficiently robust backup. Especially true when such an event does not happen with any regularity as does the weather events.
It may be cheaper for the FAA to shutdown, but how about on the airlines?
Or maybe the FAA should be liable to for paying the airlines and passengers for the problems they cause. Might have them consider other plans versus just shutting down.