Although the origins of many words are rather odd and esoteric in nature, the origins of the words “uppercase” and “lowercase” to refer to capitalized and uncapitalized letters is quite straightforward.
Back when every bit of printed material had to be handset by a printer or their assistants, the letters were arranged by frequency of use. The infrequently used capital letters went in the “uppercase”, a slanted shelf above the main work area. The frequently used uncapitalized letters went in the “lowercase”, a more accessible shelf closer to the workstation.
“I know this never kept you awake at night, but I am here to deliver worthless knowledge. So feel free to file this away in your worthless knowledge folder.”