Why is the heart located on the right side of a chest pa X-ray?

It isn't.

In almost everyone, the heart is on the left side of the chest. On a chest x-ray, however, the heart is on the right-hand side of the picture because that is where the patient's left is! It is as if the patient is standing in front of you, facing you... their right is on your left and their left is on your right.