Answer:
Well the main reason is because Buddhist don't believe in the caste system, or to be more precise they believe in equality. Therefore, they objected the caste system, as a closed and inflexible. That is why many people wanted to accept Buddhistic views.
Step-by-step explanation:
Caste system in India is very conservative. People from different castes are not even allowed to marry themselves. Member of higher caste has more rights than member of lower caste. Additionally, it is practically impossible that a member of one caste can became a member of some other caste. All of these ideas were rejected by Buddhism.