What prevented India/China from being Christianized or Islamicized?

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Essentially a lack of interest from the Turkic states that made up the ruling dynasties, who preferred to patronage and ally with certain Hindu sects; that Hinduism is a fairly decentralised faith which can survive relatively unmolested without kingly patronage (something which can not be said for the lingering remnants of Buddhism in the subcontinent) the persistence of the caste structure and the fact that India was never under any complete Islamic control, even at peak Mughal.

As for Christianity, the Brits main priorities were cash and stability, not God.

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I think the more interesting question is why did Indonesia become so islamic?