Answer:
Due to ethical reasons. Obtaining stem cells from the embryoes would require fertilisation of the embryo in order to obtain the embryonic stem cells and the embryo may not develop into a fetus upon extraction of stem cells (I may be wrong on this). On the other hand, umbilical cord stem cells, although are more mature stem cells, could be obtained after childbirth and does not raise the same ethical concerns as that of the embryo
Step-by-step explanation: