Allah Almighty has appointed Imam Ali (PBUH) with all his miracles and virtues as a leader of Islamic government after the death of the Prophet to spread the divine teachings and lead human being toward spiritual life which is needed in all the times.in doing so Allah Almighty has asked the Holy prophet to carry out this important task on the day of Ghadir.
O Messenger, deliver what is sent down to you from your Lord; if you do not, you will not have conveyed His Message. Allah protects you from the people. Allah does not guide the nation, the unbelievers.
source : erfan.ir