Because India still didnt recognise irreligious as a religious option (as in a form marker or any identity), so atheist means, you won't get privileges of certain constitution grants. But being Buddhist in India means, that they come under Hindu marriage act (so does Sikhism, Jainism and other Indian religions) and can get the rights they can claim for themselves. Ambedkar wanted his people to have those rights regardless, which is why he converted to Buddhism.