Many years ago, type was set by hand, and human typesetters kept the capital letters in an upper tray, called the upper case. The other letters were kept in a case below it, called the lower case. Over the years, as you know, all this became automated, and separate letters weren’t kept in cases at all, but were imprinted by typewriters and later made digitally. (I’m leaving out a lot of history here.)
But we kept the names for the two types of letters, and the terms “upper case” and “lower case” became adjectives, hence the change to single words, after a brief stint as hyphenated compounds. I remember when this transition to single words became official, in the mid 1960s.
All that history to give an excuse for sharing this episode of the Lockhorns: