why did lincoln inform the confederacy that he would not be sending troops or weapons as food supplies were sent to fort sumter?

Lincoln wanted the Confederacy to understand this action was not meant to be an attack, but an attempt to peaceably land only provisions.

South Carolina militia had declared that any attempt to resupply Fort Sumter would be seen as an act of aggression.

Knowing the fort was running out of supplies, Lincoln announced his plan to send three unarmed ships to relieve Fort Sumter rather than sending a military expedition to reinforce and resupply.