This is the reason.

This is the reason. [2] Indeed, there are some dimensions common to both schools. Nevertheless, there is a certain dimension particular to Shia belief. This particularity of issue of Imamate makes it an element of top priority to Shias. When we Shias want to mention principles of faith we say: Monotheism, Prophethood, Justice, Imamate and Day of Judgment. We regard Imamate a part of religion. The Sunnis also acknowledge something of a sort of Imamate.

They do not basically deny Imamate, but Imamate they acknowledge is something else in a different form. [1]…