1.The Prophet Muhammad (SAWA) was aware the he is about to leave this world, so, he tried his best to secure the future of Islam and Muslims after his departure by declaring his successors in front of Muslims, which he did while returning from his farewell Hajj in the most authentic Hadeeth of Al-Ghadeer in which he clearly announced ( For whom so ever I am the Master, Ali is his Master من كنت مولاه فعلي مولاه
2.This Hadeeth has been narrated by biggest number of authentic companions.
'The Prophet (SAWA) was aware about some persons who did not like the leadership of Ali after the Prophet,v so, he ordered them to leave Madinah and join the army if Usamah. He cursed those who disobey his order and stay back in Madinah. Many of them returned back again this orders despite his statement (لعن الله من تخلف عن جيش أسامة).
3. When the Prophet saw them back in Madinah, he ordered them to let him write his last will, which should protect the Ummah from going astray. The disobeyed him and refused to let him write his last will. This is mentioned in Saheeh Al-Bukhari and other main Sunni books.
4. When the Prophet (SAWA) passed away, some companions went to Saqeefa leaving the Prophet unbarried and made their arrangements with the hypocrites to take over the rule misusing that Ali (AS) and pious companions were busy is preparing the funeral of the Prophet.
5. Imam Ali (AS) and all Ahlul Bayt (AS) rejected Saqeefa rulers and never gave allegiance to them, which lead Saqeefa rulers attack the house of Fatima, the daughter of the Prophet and threaten to burn it in its people as you can read in Tareekh Al-Tabari and many other Sunni books.
'These are few facts from many other facts which changed the history of many Muslims from following the Prophetic Islam to following Saqeefa rulers.
'Wassalam.