This trend happened before autocomplete was common. If you logged into a commercial unix machine in the 90s it would not have autocomplete out of the box.
I assumed it was to sort at the top of 'ls' output, which is one of the answers on stack exchange.
Autocomplete was very common in the 90s — installing bash or tcsh was generally one of the very first things one did on a commercial UNIX that didn’t ship with a decent shell.
I'm having a hard time finding people writing about this, because so many people write about shells from a more recent perspective. According to this, tcsh introduced completion in 1983.
I also found in the bash changelog from 1996 there are fixes to crashing bugs in file name completion.
However, I stand by what I said. It was common for people not to know about filename completion. It was common to ship without it by default. Most clued in people installed bash on those systems. But not everybody.
