Answer
The stem cells from an embryo are pluripotent, allowing them to replace damaged or diseased tissue in a patient by creating wholly new healthy tissue.
Work Step by Step
Your answer may differ. The scientific argument for using aborted fetus tissue is that this tissue is more likely to be healthier and would also be more fresh, increasing the odds of a successful transplant. A fetus that has spontaneously died in utero may have had anomalies that would make the transplant less likely to succeed. The tissues may also be older and less likely to survive in the recipient. From a purely practical standpoint, it would be more difficult to obtain viable tissue from miscarriages.