Imamat exclusively for Ali (a.s.) due to his perfect Tauheed
There are many conditions for being a divine proof – be it a prophet or a successor of prophet. Among these prerequisites is perfect tauheed – which means that the divine proof should never have worshipped any deity other than Allah, even for a fraction of a second.
Regards successorship of Holy Prophet (s.a.w.a.), there was no one better placed than Ameerul Momineen (a.s.), Ali Ibn Abi Talib (a.s.) on the all-important criterion of tauheed. No other companion (with possible exception of Salman al-Muhammadi) can claim perfect tauheed for himself, which makes them ineligible for leadership of the nation.
An interesting report on the authority of the Holy Prophet (s.a.w.a.) establishes this fact clearly:
The Prophet (s.a.w.a.) informed his companions like Abdullah Ibn Masood:
I am the prayer of my grandfather – Ibrahim.
Companions: How is that?
Prophet (s.a.w.a.): Allah revealed to Ibrahim (a.s.) – Surely I will make you an Imam of men.
Ibrahim (a.s.): Will my children be Imams like me?
Allah revealed: I will fulfill every convenant I make to you.
Ibrahim (a.s.): Which convenant?
Allah: I will not grant Imamat to oppressors in your progeny.
Ibrahim (a.s.): Who are the oppressors in my progeny?
Allah: One who prostrates to idols instead of me, I will not grant him Imamat ever nor is it appropriate for him to be an Imam.
Ibrahim (a.s.): ‘and save me and my sons from worshipping idols. My Lord! Surely they have led many men astray (Surah Ibrahim (14): 35-36)
Prophet (s.a.w.a.): Ibrahim (a.s.) prayer (of saving his children from idol-worship) concluded in me and in my brother Ali (a.s.), none of us ever worshipped idols. So, Allah chose me as prophet and Ali (a.s.) as my successor.
Al-Manaqib of Ibn al-Maghazali p 276
Shawahid al-Tanzeel v 1 p 411-412
Al-Amali of Shaikh Tusi p 379
Kashf al-Yaqeen p 412
Al-Taraef p 78
Al-Jawahir al-Saniyyah p 514
Evidently, with his perfect tauheed in Allah, having never worshipped any other than Him, just like the Prophet (s.a.w.a.), Ameerul Momineen (a.s.) was the only one fit for successorship of the Prophet (s.a.w.a.). By rejecting Ali (a.s.) for others who worshipped deities other than Allah, the Muslims have mocked Tauheed of Allah and those who in their entire lives never worshipped any deity other than Allah.