Ambedkar was looking for a religion for lower-caste and dalit Hindus. In doing so, he evaluated every religion in social terms (and not based on beliefs). So, Hinduism was a no to him.
But so was other religions like Islam, Christianity and Sikkhism, in which he predicted Dalits, as new converts, would still have a secondary status as compared to those Indian people already in the religion.
This is the reason he chose Buddhism, because the religion had disappeared from India altogether. Hence, dalit converts would not have nil status to a pre-existing group, rather it would be a brand new community.
His primary reason for rejection of Islam and Christianity was that it would result in the “de-nationalisation” of Dalits. Islam and Christianity are foreign religions of a place far away from India, they’re not Indian. Sikhism was Indian, so he researched more on it, but rejected due to the secondary status. Thereby, settled on Buddhism.
Source : Reddit India