Many reasons I think. Buddhism is a world religion, but Hinduism is more like an Indian religion. Also, Buddhism is a modern religion, based on a single prophet or preacher (just like Jesus in Christianity, prophet Mohammed in Islam), while Hinduism is a complex set of ancient and diverse beliefs. Buddhism is gaining a lot of followers world wide, through outreach activities. Hindus never believe in any outreach / missionary activity, and are normally against other people joining their religion (with some notable exceptions). Also, most importantly I heard that a Hindu can be an atheist as well.