Please explain reasons why transposition of the great vessels is deadly?

Abnormal connections: In tga (transposition of the great arteries) the aorta arises from the right ventricle and the pulmonary artery arises from the left ventricle. In this arrangement, oxygenated blood cannot reach the body, so oxygen delivery to the body is severely impaired. While there are treatments that can stabilize the circulation for a few days, an operation to "switch" the arteries is required as a newborn.