Well, in UNIX and shell, variable case is relevant, $HOME is not $home or $Home or $hOME, and some are very old choices, old like COBOL 68, before computers could afford lower case, so upper case is traditional for all env variables used by O/S or compiled code and other shared applications.
In your own private shell scripts, feel free to use lower case for your own data, without fear that you are accidentally turning on the $GO_AWAY_FOREVER option.
I can still hear those mainframers at AT&T, when I suggested encoding a flag byte with lower case for additional situations, mnemonicly, growling "You UNIX guys and your lower case!"