18th Century Revolts And Reform
The main cause for the Instant popularity of the Arya Samaj was that

It represented the purified form of Hinduism
It gave a call to its followers to go back to the Vedas
It combined the opposition to the evil practices of Hinduism with an aggressive assertion of the superiority of the Vedic religion and Indian thought over all other faiths
It supported and worded for the cause of western education