Why is the word I capitalized?

The pronoun I /aɪ/ is the first-person singular nominative case personal pronoun in Modern English. It is used to refer to one's self and is capitalized, although other pronouns, such as he or she, are not capitalized. The grammatical variants of I are me, my, mine, and myself.

It is a pronoun.  However because of the look of the letter "i", it was difficult for scribe to write as a single letter in lower case and have it stand out.  So it became a tradition that it is written as a capital letter.  That is still consider proper English today.