Because frost means heat has already reached the outer surface, which means the vacuum has already gone.
A sound vessel has a cold inner vessel and an outer jacket close to room temperature, separated by an evacuated space. Frost on the outside means that separation has failed enough for the shell itself to have chilled below the dew point. At that stage the vessel usually needs to come out of service, and you are managing an incident rather than a maintenance item.
The warnings arrive in order. On a pressurised vessel the pressure trend moves first, because pressure responds to the rate at which liquid is turning to gas. Consumption in litres per week moves next. The level gauge follows. Frost is last.
Which is why a logged weekly consumption figure is worth more than any single inspection.
