What groups of people were responsible for the large conversion rates when Buddhism first emerged

Vedic faith: In the era after 1500 BCE it is believed that the Indo-Aryans community had beliefs based on the teaching of a very ancient faith called as Vedism, which is also considered as the predecessor of Hinduism. As the folk living in the region of India and most of Europe had beliefs relating to Vedic faith.

Explanation:

When Siddhartha Gautama started preaching of Buddhism, it is believed that in that era the people in the region were no more following the Vedic faith, as most of them disapproved the Vedic faith. They wanted a new way to live there lives. For that Buddhism provided a more peaceful and flexible set of faiths to spend their lives with, as they were very much convinced by Siddhartha Gautama teachings so they converted into Buddhists.