есть еще одна версия by Chuck Bigelow
On the other hand, perhaps, as some have suggested, those 14th century
scribes just didn't like the look of that puny little "i" (often without
even a dot capping it) all by its lonesome out there on the page, so
they used capitalization as a grammatical marker to signify that the
letter was a word, and an important kind of word. (We don't need to
speculate on the egotism of the English. It's enough to acknowledge the
grammatical importance of the nominative singular pronoun as compared
to, say, the indefinite article 'a'.) Here, the scribes would be employing
another use of orthography, to mark a word class in a special circumstance.

Почему? =)

А я тоже не знаю... Почему?

вот..с какого - то форума..уже не помню точно откуда :)
удивительно просто!

На самом деле надо обратиться к истории английского языка.
В древнеанглийском периоде (до конца XI века) местоимение "я" выглядело как ic. В среднеанглийском периоде (до XV века) добавилась буква 'h', т.е. "я" стало ich, через некоторое время "ch" отпало, осталось только i. И поскольку тогда слова писались без пробелов, чтобы не потерять элемент i, и чтобы не подумать, что это часть другого слова, стали писать I. И уже с XVI века так и закрепилась эта традиция - "Я" в английском языке пишется только с большой буквы.

The Barnhart Dictionary of Etymology (Wilson, 1988, ISBN
0-8242-0745-9) says: "~I~ pron. 1137 i; later I (about 1250,
in The Story of Genesis and Exodus); developed from the unstressed
form of Old English (about 725) ic singular pronoun of the first
person (nominative case). Modern and Middle English I developed
from earlier i in the stressed position. I came to be written
with a capital letter thereby making it a distinct word and avoiding
misreading handwritten manuscripts. In the northern and midland
dialects of England the capitalized form I appeared about 1250.
In the south of England, where Old English ic early shifted in
pronunciation to ich (by palatalization), the form I did not
become established until the 1700's (although it appears
sporadically before that time)."

oooo