The answer is buried in a history of the English language text—Pyles and Algeo, Origins and Development of the English Language, 4th ed., p. 187:
"I" came to be capitalized, not through any egotism, but only because lower-case i standing alone was likely to be overlooked, since it is the most insignificant of the letters of the alphabet.
This is found in the chapter on modern English to 1800, meaning in the period from 1500 to 1800. Thus, I take this to mean that the capitalization of I for the personal pronoun was a printer's invention during the early part of this period. I do know that in manuscripts from the Old and Middle English period, the forms were "ic" (OE) and "ich" (ME) and were never capitalized in script.
— discovered by
Dale W. Simpson
Professor of English
Missouri Southern State College
Joplin, Missouri
"Origins and Development of the English Language" by Pyles and Algeo
4th edition, p. 187:
"I" came to be capitalized, not through any egotism, but only because lower-case "i" standing alone was likely to be overlooked, since it is the most insignificant of the letters of the alphabet.
This is