Remember when typewriters were a thing?
Notice that everything is in UPPERCASE, because the the very first typewriters ( telegraph teletypes ) did not have lowercase at all.
Also there were no CTRL, ALT meta keys then.
The reason that historically meta keys were always UPPERCASE is in the old days pre-90's. It was the way books and ASCII text based systems distinguished a special KEY from text that you actually typed in. And they would usually be surrounded by brackets as [CTRL],[SHIFT], [ALT], [LSHIFT] and so on.
Also you usually represent the individual keys with UPPERCASE to help distiguish an I from l or 1 in older systems because the fonts were terrible at the time. If you wanted to someone to actually type an UPPERCASE character you documented it as SHIFT+L or SHIFT+I or SHIFT+A because if you look at the keyboard keys themselves they have been in UPPERCASE for decades just for this reason.
So look at some printed manuals/documentation, it was a thing one, and you will quickly see why the proper way to indicate something would be CTRL+ SHIFT+DEL from the DOS era and not what the keyboard actually had, because all the keycaps representations varied.
It has never had anything to do with the representations on the keyboard itself. It was all about disambiguate what you literally typed from what meta keys you pressed and held.
So CTRL is how the documentation for the <kbd/> tag is represented in the markdown documentation.