Airstrikes don't deliver food and water. An airstrike is an attack on enemy assets (troops, equipment, infrastructure, etc.) Delivering food and water would be an airdrop. Obama has done both (as have other countries). They have airdropped supplies to the trapped people and have attacked enemy assets to try and prevent them from being able to attack the people. So far it seems to have had minor effectiveness. Some of the Yazidis were able to escape, and some enemy assets were destroyed, such as an artillery piece which had been bombarding the mountain, but overall it has been too little too late.
Those who escaped report severe shortages of food and water, so it is likely that whatever was dropped was consumed pretty quickly, and therefore probably wouldn't be sitting around for ISIS to go get.
Additionally, the planes dropping supplies are large, with jet escorts. And while I don't know if the supplies dropped had parachutes or the altitude they were at, regardless, it isn't much of a stealth operation, and I suspect would most likely be noticed regardless of Obama broadcasting it or not.