It's an idiomatic form that defies analysis. ", but, I'm not sure. I think it originated in America, so maybe an AmE speaker can help more.
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MapleDoes "come" function like "do" in "How come you vote not guilty", or "vote" is an infinitive whose "to" is omitted?"How come" is an idiom expression which means "what is the reason" or "why". It's frequently used in everyday life.