Allah knows the wisdom behind every thing He ordained. We believe in the Absolute Wisdom of Allah and do not go in to details which is out of our reach of thinking. Our Faith in the Absolute Wisdom of Allah (SWT) is enough for us and it is the real and proper way when we deal with decisions of Allah ( SWT).
We and all the scholars might thing about some reasons like the reward of the sacrifice in Karbala, but that can never be the full reason.We always say: Allah and His messenger know more. الله ورسوله أعلم
The question Why when it comes on any rule from Allah can mislead people who do not have enough submission to the will and wisdom of Allah (SWT).
In the life of all the prophets and infallible Imams, similar question about reasons of succession can mislead weak hearted persons who don’t have real faith in the absolute Wisdom of Allah (SWT).