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Megagram is the official SI term for the weight. Metric tonne is non-SI but happens to be equivalent to a megagram and became the more common parlance (where I am, at least) by historical accident.
Pretty sure they tried to mimic existing units/terms to make it "easier". So they used tonne to mimic ton.
They didn’t mimic existing units, an imperial ton is close to a metric ton, and the spelling tonne is just an alternative spelling of ton. In some parts ton means imperial ton, and tonne means metric ton, but it’s not standardized. In German, where the word originally comes from, it’s Tonne (btw the e is not silent, it’s [ɛ] as in let. Or in Porsche (no, it’s not pronounced porsh…).)
They mimicked existing terms, otherwise we wouldn't have ever had the term metric tonne. It would have been called a megagram.