Because that is when the DOI was signed, it was passed later on I believe
I believe the answer is: order in society
Religion usually explicitely indicate the things that must and musn't be done by the followers so they can obtain some sort of reward in the after life. Confucianism on the other hand, only invite people to focus on self-development and respect one another so they can create order in society.
A perpetual set is the answer.
I hope this helps!