Why is the letter "I" (pronoun) always used in uppercase

I don't think there's any particular grammatical rule. We don't capitalize the "m" in "me" or the "h" in "him" or "her".

I think it's just simply a rule. It probably came from the German where they capitalize all nouns (if I remember my high school German correctly). "Ich" means "I", and the old English probably changed it to simply "I", and it stuck.

The reason why is to make it distinct from the roman numeral I. In Old English the word was ic, and in Middle English it was ik, iche, ich, y, and I. This source can help explain the reason more:
Online Etymology Dictionary

Its on of those strange rules English The Easy Way

I is the first person according to grammar. Always used in upper-case.