Why is "I" capitalized?

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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)."