Because Lincoln's clearly stated policy was to maintain federal property without causing friction. The fort had -been- stocked before; restocking it was not an act of war. Firing on Old Glory was and is.
Walt
The South has furnished near three-fourths of the entire exports of the country. Last year she furnished seventy-two percent of the whole ... we have a tariff that protects our manufacturers from thirty to fifty percent, and enables us to consume large quantities of Southern cotton, and to compete in our whole home market with the skilled labor of Europe. This operates to compel the South to pay an indirect bounty to our skilled labor, of millions annually.
Daily Chicago Times, 10 Dec 1860
That either revenue from duties must be collected in the ports of the rebel states, or the ports must be closed to importations from abroad ... If neither of these things be done, our revenue laws are substantially repealed; the sources which supply our treasury will be dried up; we shall have no money to carry on the government; the nation will become bankrupt before the next crop of corn is ripeThat's why Lincoln rushed to restock. And so much for Llan's assertion that the South leached off the North - It was the other way around.
New York Evening Post, 12 Mar 1861