According to Wikipedia, there is no known record of a definitive explanation from around the early period of this capitalization practice, though there is scholarly merit (and simple curiosity) to prompt the continued seeking of an explanation.
It is likely that the capitalization was prompted and spread as a result of one or more of the following (in alphabetical order):
Changes specifically in the pronunciation of letters (introduction of long vowel sounds in Middle English, etc.)
Other linguistic considerations (demarcation of a single-letter word, setting apart a pronoun which is significantly different from others in English, etc.)
problems with legibility of the minuscule "i"
- Sociolinguistic factors (establishment of English as the official language, solidification of English identity, etc.)
There is also the possibility that the first instances of capitalization may have been happenstance. Either through chance or a sense of correctness, in the practice or the delivery, the capitalization may have spread.