No, there is no explanation. The explanation is "just because." Maybe you could find some history of written English that goes beyond that, but I don't know of an explanation _and I would not necessarily believe it_ if I found one. It is the same kind of question as "why does February have 28 days?" or "why does French have irregular verbs?" "Are the rules regarding this pronoun changing?" No, they are not. Because it is somewhat awkward to type uppercase letters, particularly on cell phones and touchscreens, there is an increasing tendency for people to type everything in lower case, with no capital letters at all. The rule for "I' isn't any stronger than other capitalization rules. If someone is typing everything in lower case, they are going to type "I" in lower case as well. I don't know if this is eventually going to change the written language. I have not seen any signs of it so far. For some reason, the US poet e. e. cummings wrote everything entirely in lower case. If you look at this page, you'll see a poem in which he uses the word "I" in lower case throughout. That was unique to e. e. cummings. Nobody else does it. https://www.poetryfoundation.org/poetrymagazine/browse?contentId=22656

It is always proper to use the capital I. Always, no exceptions. The lowercase i is used only in informal writing, like in text messages. It is best to always use the uppercase I.

I think it is easy to intuit the reason why "I" is capitalized. "I" is simply the most important thing in the world or language, so it gets the special status. The world exists because I exist first to perceive and understand it. If I don't exist, there would be no world, no God, and no human race of over seven billion. "I" is the single most important concept, the start point of everything, so to me it seems natural that "I" is capitalized.

thank you John for the historical perspective.

it's a relic from the past, and there are many theories and explanations. That cover a whole range of topics. from "ich" in German on English coins etc, and from the fact English has Germanic roots. Ranging to the invention of the printing press. Where it was needed to determine difference between a single stray "i" or "i' meaning the person writing or speaking. At some stage in the English language history it was determined and possibly needed to clearly know when "i" meant a single "i" or it meant "the person I". This apparently used to happen at the beginning of a new line of print. But it seems bizarre. The point is it has become set in stone. And now you have to do it always for Exams and formal writing. For native English writers it is now natural and instinctive to Capitalise when using a PEN. When typing it is the most common typing mistake, because to type a "I" you have to hold down another key. You are right it is not actually needed any longer. it is a mistake I make sometimes when TYPING