Hi @damzxyno, good question.

Cellular rejection is caused by the immune system and in such an early stage of development (called blastocyst stage) the cells have not yet differentiated to form different types. So not only is there no functional immune system, but there's not a single cell in either of the groups that would act as an immune cell. As such they do not recognise eachother as 'different'.

Great question, I hope my response makes sense!