I can see the reason from Beauregards angle. In one of the letters from Anderson to Washington in December or January, he said he had enough food and supplies for a year. That is why I wondered why the shortage became desperate, so to speak, or were 'new supplies' the ruse we have been thinking it to be? Were 'reenforcements' part of the supply list?
Or course no one wished to ruin a perfectly good fort if they didn't have to. It would be useless.
Pam