Which populations? In Java, Cambodia and South India? They took Hinduism because very alien peoples of North India (culturally, linguistically, sociopolitically, spiritually, genetically, even in physical appearance) just grew in population thousands of kilometres away?
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Any articles on the indianization of SEA state something roughly similar to those statements: Hinduism arrived to the very alien cultures and languages of SEA via "traders, adventurers, teachers
and priests". Khmer king Jayavarman II founded a cult which literally identified the king with the Hindu God Shiva, and Angkor Wat was originally devoted to Vishnu with all its motifs derived from Hinduism and "correctly" applied. You don't get seventh wonder of the medieval world, so to speak, erected in the name of your faith by completely alien people like two thousand kilometres and an ocean away from your furthest boundaries... without some kind of missionary activity. I assume at one point proto-Khmer rulers had to get the entire Vishu, Shiva, Brahman and Upanishad stuff explained to them by some very hard-working priests from India, they couldn't read about it on the Internet and then spawn their own Khmer-speaking Hinduist priests and philosophers ex nihilo