Well, that. Why don’t use ‘You’ in uppercase?
Just egocentrism? 
Is it wrong to write ‘I’ in lowercase?
Two possible reasons:
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Unlike other pronouns, the word “I” can only refer to one possible person/place/thing – the speaker. Therefore, it’s treated as a proper noun. “You” could refer to one or more persons the speaker is addressing.
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a lower case “i” is too easily confused with a typo.
It shows you do not have an low self-esteem.
It is grammatically incorrect to write “I” in lowercase, although in some contexts you can plead poetic license.
As for why, well, for no real reason that I know of. It’s just a quirk of the English language.
Being a native English speaker, I never put much thought into this. Perhaps it’s because in modern English there is no switch between speaking formally and casually. I’m guessing the capitalized I would then be a leftover from Old English, which did have that function. When I first started learning German, I was perplexed as to why “ich” was never capitalized, unless beginning a sentence. Seemed sort of degrading. However, it inevitably calls upon the tune of 99 Luftballoons by Nena…
“Hast du etwas Zeit fuer mich, dann singe ich ein Lied fuer dich?”
(Do you have some time for me, (so) that I can sing a song for you?)
I think I’ll pass. 
But “Du” is capitalised in German, if you are writing a letter.
Also, when referring to God, I was taught to write “He”, “Him” and “His” with capital letters. I don’t know whether this qualifies as a grammatical rule or a religious one (does it apply to writers of English who are not Jewish or Christian)?
IANALinguist, but…
I believe at one time all nouns in English (or proto-English) were capitalized. Somewhere along the line that fell out of practice except for names and places. I’d imagine that they kept the capitialization of “I” because “i” looks funny.
I seem to recall reading somewhere that the capital I is a survival of the medieval “long i” — nowadays represented in French by the j in the first-person pronoun j. In other words, it’s a remnant of French influence upon English, and not a mark of supposed English egotism.
While the French j extends below the line, English in the Middle Ages hadn’t yet caught on to writing j like that in uncial hand. Maybe the English used a different style of uncial. So the way to show “long i” was to extend it taller. Or something like that. (To this day, in Italian, the letter j is named i lungo. The Italians formerly used it as shorthand for double ii, but now they don’t really use it any more. Why does Washington, DC have I and K Streets, but no J Street? Because in the 18th century, J wasn’t recognized as a letter of the alphabet in its own write, it was just a variant of I.)
But note that there’s only one other single-letter word in English: O. The vocative “O” as in “O mistress mine, where are you roaming?” It too is always capitalized. Coincidence?
Maybe the English scribes just felt that a single-letter word all by its lonesome looked too puny without capitalization. The Italians never considered it a problem; they use five single-letter words of high frequency: a, e, è, i, o. In English, these being comparatively rarer, maybe the scribes felt some need to buff up their size. Just my WAG.
Similarly, in late medieval/early modern English, 2-letter nouns were felt to be too short to be proper words, so you get a third letter added: for example, add, egg, odd. They could just as well have been spelled ad, eg, od, but somehow they looked too small for nouns; all the other nouns have 3 letters minimum.
I believe you made a mistake.
Well, apart from that, OK?
It seems the English felt fine about the dinkiest little word in the language staying lowercase; it’s just a schwa and a garden variety “function word”. Maybe because it normally doesn’t take any stress emphasis i spoken English, while I and O often do.