The short-run aggregate supply curve is relatively flat to the left of the full-employment
output because resources are difficult to bring into production.
there are shortages of capital.
there are large amounts of unused capacity and idle human resources.
there are shortages of labor.

There are large amounts of unused capacity and idle human resources.

Explanation:

The curve is flat because there are large amounts of unused capacity and idle human resources. When the economy is operating to the left of full employment it means that is not utilizing all it's resources and not fulfilling its potential, because of this the economy has lots of idol capacity and resources and they are available for cheap, the reason for this is that the resources are abundant and thus very cheap so this makes the left of the full employment short run aggregate supply curve relatively flat.

    In the midst of an atomic war on Earth, the United States sends a number of colonists to establish an outpost on Mars. The Bittering family, father Harry and mother Cora and their children Dan, Laura, and David, arrives as part of the eight hundred colonists chosen for the first wave. Harry is initially disquieted by the Martian environment, but takes comfort in the fact that the family can return to Earth when resupply ships arrive.

    Strange events begin to affect the life brought as part of the settlement effort, including the seeded grass sprouting purple, the family cow growing a third horn in the middle of its head, and other anomalies with the vegetable garden. Harry's discomfort on Mars increases, and the thought of returning to Earth on the next resupply mission soon becomes his only comfort, much to the concern of Cora. This comfort is taken away as Bittering is informed that the war has led to an atomic bomb devastating New York City and destroying the only spaceport capable of supporting travel to Mars.

    Resolving to build himself a rocket home, Harry isolates himself from his family and the townsfolk, who have begun to show signs of transforming into Martians as their limbs and bodies elongate, their irises become shimmery gold, their skin darkens, and they begin using Martian language, referring to Earth as Iorrt, the ancient Martian name for Earth. Harry staves off the transformation as he only consumes food and water brought from Earth, but the supplies run out and he is forced to eat Martian food to survive. Soon enough, Harry notices his eyes have turned gold. Cows are growing 3 horns.

    Cora convinces Harry that a family swim in the canals of Mars would do him good to relax, and he hesitantly agrees. While there, their eldest son, Dan, requests to be referred to by the Martian name Linnl. Harry and Cora, now almost entirely Martian, agree easily, and the other two children quickly adopt Martian names as well. As they return to the town, the Bitterings discover that the colonists are retreating to the ancient Martian villas in the mountains, as the summer has made the valley stiflingly hot. Harry briefly expresses a wish to stay and work on his rocket, but is easily persuaded to go with the rest of the colonists and come back when the weather is cooler.

    Five years later, the United States, having won the war and rebuilt New York, sends a small military dispatch to recover the colonists sent to Mars, only to find their settlement abandoned. The soldiers instead encounter a large Martian settlement in the mountain villas, where the native Martians are pleasant and have a remarkable affinity for English. Convinced they had nothing to do with the original colony's disappearance, the group agrees to attempt a second, larger settlement using the town built by the first.