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I'm not sure there is a "reason", other than usage and convention.
In some very early texts, you do find a lower case "i" sometimes. Perhaps the original reason was to distinguish the first person singular from the numeral "1", which was often written "i" in early texts. (But that's only a guess!)
(I'd be interested to know whether any other members know more about the "majuscularity" of "I".)
Why "I" and not "i" ? 
Linear Mode
