From the beginning of calling for his honorable message, the Prophet (pbuh) was keen to clarify and confirm the authority of Ali (a.s) and being the successor to him. He didn’t skip any occasion or suitable opportunity to talk about al Imam Ali, his spiritual authority over muslims and the necessity of following him. The important paradoxes are that the honorable hadiths that narrated these allusions were followed in succession among all Muslims. The claim that Muslims did not agree on anything as they agreed in the narration of the virtues of Ali (a.s), and his worthiness to the Imamate after the Messenger of God (pbuh), is not aimless even if they differ later on how to interpret and direct it, in line with the existing policies after they were unable to deny it.
The holy Quran has also mentioned the authority of Ali (a.s) in many holy verses that don’t leave any doubt that he has the authority after the messenger of God (pbuh) through realizing conditions such as the surrounding circumstances, reasons of revelation, verbal connotation and others.
But, the question that is still asked is why isn’t his name clearly mentioned in the holy Quran if his Imamate has the same importance of the prophecy, so that the unbelievers and those who have diseases in their hearts can take enough evidence.
The answer is, God knows best, is that after mentioning the traits that a guardian should have. There is no need to mention his name (a.s) because the Islamic message is not circumstantial and temporary and its effects don’t end with the end of conditions. It is general, inclusive, permanent and fixed until God almighty inherits Earth and its creations. The Imamate is not limited to Ali (a.s), but fixed to his children the twelve Imams (a.s) after him. Then, if his name was mentioned, it was necessary to mention the names of his children; but, God almighty knows that the surrounding conditions will oblige the twelve Imam (Al Imam Al Mahdi) to disappear from the eyes of people for a long time, and that the believers will be obliged to manage their issues by themselves. This point necessitates people to figure out the traits that should be present in the man who will take the Imam’s place and work on accomplishing them during his disappearance. In addition, Ali and his children (a.s) constitute the complete evidence of these traits and conditions to prove for all those who know these traits and look at people’s issues that they are the only ones who have these traits, if they are present. This is from the first side.
From the second side, Ali (a.s) had many enemies from Quraish and other Arabs, those who surrendered because of fearing from his sword, those whom he killed their courageous, knocked down their heroes, and who hope to monopolize the prophets heritage in order to get the authority, prestige and else. This made him behave with him with enmity and envy because of what he’s bestowed of God’s blessings. Then, if his holy name was clearly mentioned in the holy Quran, they would reject Islam and embracing it, especially those whom Al Imam Ali (a.s) has killed their men and humiliate their pride.
Because the true Islam is the religion of guidance and mercy, it wants all of people to be attached to it, behave according to its teachings to purify their souls and refine themselves. It wants to close all of the doors and damage the struggles that prevent their guidance so when they achieve this level of goodness, they can follow Ali and his children (a.s), unconcerned with their enmities and hatred.
In another expression, Islam wants to put people in the beginning of the right path, to cross it alone by using their brains to continue through the clear and reasonable way. (This is My exact right path, so follow it, and don't be diverse, lest they should deviate you away from His (straight) way.' That is what He enjoins you with, that you may guard against evil). 
On the third hand, there is no difference between the concept of the Quranic verses and the prophet’s hadiths since “nor does he (the prophet) speak out of desire, indeed it is not except a Revelation which is revealed”.  Hence, the words of the prophet are the same as the Quran words and vice versa with a simple exception in this issue. If that issue was clearly mentioned, it would be a motive for many to shove away from God’s path, but it may also push them to manipulate Quranic verses by deletion and distortion to get rid of this problem. God almighty wants to protect Islam from this. But mentioning this issue through the prophet Mohammad (pbuh) will give them an extent between themselves to accept the message from one side and push the authority from another side until they believe in the evidence and believe in him and in his authority. Therefore, the importance of the coming generation to be guided to the truth is by turning back to this issue and carefully looking at its concept after being sure that it is truly revealed without any manipulation or distortion.
In addition, their recount of the hadiths that show the favor of Imam Ali (a.s) and his position in the eyes of God’s messenger (pbuh) strengthens the evidence with a confirmed concept after they admit that by their tongues.
His Eminence Al Sheikh Hatem Isma’il, The Principle in Quran
 Surah Al An’am, verse 135
 Surah Al Najem, verses 3 and 4