My guess is the tty culture on UNIX revolved around the Model 37.     This had upper and lower case as well as having the concept of NEWLINE as both the shoot the paper up and move the type element back to the left side.    It ceratainly is ingrained into nroff, all those ESC-8/ESC-9 stuff that nroff puts out are direct commands to the Model 37.

I suspect that it was just the case that most PDP consoles were Model 33’s.   They were cheaper and pretty ubiquitous.   Even when I started with UNIX in 1977, most of our machines had Model 33 consoles even if they had CRTs or fancier printing terminals (LA36 or Model 37’s).