Well, since the current top comment just copy-pasted the New York Times without actually including the part that answers your question, I’m gonna put this in a top-level comment.
This is based on this article: https://www.nytimes.com/2008/08/03/magazine/03wwln-guestsafire-t.html
Basically, according to the historian in the article, single-letter words just looked weird to scribes in the olden days—like you made a mistake while writing or forgot the rest of the word.
The historian doesn’t directly explain why “a” is okay lower case, but that might be because “a” is a much less “important” or “weighty” word. He suggests that, for those old scribes, “I” had too important a meaning to be written so small, so they just made it bigger, until it settled into being upper case.
It’s also worth noting that, in Old English, “I” was not a single letter, and used to be “ich” or “ic”, kinda like German (which English is in the same language family as). It got shortened over time.
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