I think this makes more sense (or at least gets more directly to the point) than any of the Stack Overflow answers. Lowercase is easier to read in print as our minds learn the shapes of the words and can interpret whole words at a time rather than letter by letter. But Morse code was originally transcribed by hand, and it is easier for sloppily written lowercase letters to be mistaken for one another than the more distinctive uppercase letters, so it became a standard to write in uppercase. This tradition carried on to teletype and early terminals until both cases were supported.