It's a holdover from the days of Morse code. Recall the first computers were for military problems, and the first output was teletype. Teletype was originally for military messaging and that had a long history of using all caps because they relied on manual transcription of Morse code (and other codes) over wire and radio. The all-caps policies were put in place to make sure the officers could consistently read what the operator had transcribed. Some of these date back to the 1850s. The Navy didn't actually do away with all-caps until 2013.
https://www.al.com/wire/2013/06/navy_puts_all_caps_communica...
https://www.doncio.navy.mil/chips/ArticleDetails.aspx?ID=489...