Why is it that the correct placement of CPR is in the lower half of the sternum?

Actually, the correct hand placement is more in the center of the sternum. You want to avoid the xyphoid process which is at the end of the sternum, if one is not present, which is common, it's even more crucial to avoid the end of the sternum for risk of compressing and puncturing vital organs such as heart, lungs, diaphragm, and liver. Hands should be placed in the center of the sternum approximately at the nipple line. You place your hands there because it's anatomically correct in relation to where the heart is, which is what you're wanting to compression to produce blood profusion.