BECAUSE Davis knew a warship was approaching with TROOPS & ARMS and SUPPLIES......DUH!
It was not a warship, it was a merchant vessel and it only carried provisions. If they had simply let it land, Anderson and his men would have sat there for another 30 or 60 days until they needed more supplies.
How long could they have sat on that rock before they finally gave up? Who knows for sure, but Jeff Davis knew damn well that Anderson and his 65 men were already fast becoming heroes throughout the nation (and not just in the north) and the more they were admired for their perseverance, the worse it made the radicals in Charleston look.
PR-101. When it starts looking bad for you side, do something -- anything -- to change the equation. For Davis, that was start shooting.
Let me ask you. Do you think order to fire on Charleston was based on Lincoln's decision to resupply the fort?