I imagine it is economies of scale; even with Hindoo labor cheaper than Brittish (though remember that many working Britons at the time were themselves little or no more than slaves), the new big industrial furnaces were a cheaper/more efficient way to smelt than traditional hand processes. Also, this is how England tried to run colonies; raw material out; processed goods in. How did iron shift from being a raw material to a processed good? Allow me to suggest two factors; 1/ the Hindoo iron may not have been consistent enough or otherwise "good" enough (which often boils down to too alien, when humans say it; "I don't understand it, so it's stupid....") by Brittish industrial standards to be used as a raw material in their industries, and (related) 2/ the English steel and iron production had become both controlled enough and standard enough (not to mention copious enough) to produce products famed and used throughout the world.
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