Using all caps goes back to the very first RFCs, so they probably stick to that for historical reasons even if the modern convention is lowercase.
- THE INTERNET STARTED IN THE 1960'S.
- THE FIRST VIDEO DISPLAY TERMINALS THAT CAME OUT IN THE 70'S DIDN'T SUPPORT LOWERCASE, E.G. THE DEC VT05.
- PHYSICAL TELETYPES AT THE TIME ALSO WEREN'T GUARANTEED TO SUPPORT LOWERCASE, LIKE THE DECWRITER LA05.
And don't get me started on punch card programming in college in 1980 :)
(I tried it on a regular Linux virtual terminal (as is my normal way—I don’t use a graphical login manager), with my password in correct/inverted/upper/lower case, none work. It’s conceivable a pam module could make it work or something like that. I’m not investigating.)
getty -U