Because he was a social fighter and he led the reformation in hindu religion

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Raja Rammohan Roy was one of the greatest social and religious reformers of the 19th century. He founded the Bhramo Samaj in 1828 which believed in Monotheism or worship of one god. 
He discouraged child remarriage and advocated right of widow to remarry.
It was because of the efforts made by raja Rammohan Roy thta, William Bentinck, the Governer-general of India, passed a law in 1821 making the practice of  Sati illegal and punisheble by law.
He was  a great personality, who belonged from bengal.

hope it helps.. :)
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