The historical circumstances prove that a few dominant Emigrants disputed over the caliphate. That is why the Helpers hurried to Saqifa, because they tried to negate the Emigrants agreement. Perhaps, these Emigrant companions were not sure about the successor of the Prophet (PBUH), but they were firm to deprive Imam Ali (PBUH) of his right.
If there is a large body of evidence to prove that Imam Ali (PBUH) was the real successor, why he did not become the Caliph immediately after the Prophet (PBUH)? The Helpers knew that some senior Emigrants had disputes secretly over the caliphate, so they hurried to Saqifa to discuss the issue, and to keep their authority as the Helpers of the Prophet (PBUH) who housed him. It shows that there were attempts to take Ali’s right. One of the most important reasons that sought to deprive Imam Ali (PBUH) of his right was the Arab’s enmity toward him, because he had killed many of their relatives in the Battle of Badr and Uhod. Indeed, they took revenge on him for killing the Arab heroes. [1] Moreover, Imam Ali (PBUH) was busy doing the Prophet’s burial when some senior companions gathered in Saqifa electing a caliph against the Messenger’s will and order. Umar, who became the second caliph afterward, played the prominent role in Saqifa, then he forced people to commit allegiance to the first caliph.
[1] Sharhe Nahjo Al-Balaghah,Ibn Abi Al-Hadid, Vol 9, P 23