An ovum and a donor cell are very different types of cell. The completely nucleus is placed in a denucleated ovum to create what is essentially a zygote that will have the correct cell biology to go on and develop into a full organism.
I think the OP is trying to ask why the host egg cell was taken from a different species of sheep and not from the same species or even the same animal from which the donor nucleus was obtained.
I don't have the answer atm; need to do some research...