If you put an OLED display behind dark plexiglass it looks really nice -- the border of the display becomes invisible and you see just the letters (or whatever else you are displaying).
I'm not part of the team that makes such decisions, being a lowly software engineer that never got any good at EE. However, as one who was at least in the room when such decisions were discussed, I believe the decision-making process consisted of "OLED would be way cooler, and they're not that expensive these days". I'm pretty sure that is the sum total of that "engineering" decision. :-)
But I will say that the OLED looks a lot better than most LCDs of that type that I've seen.
EDIT: oh, yeah; sibling comment says something about backlighting an LCD, which might have had something to do with it, as the OLED replaces a seven-segment LED display from the previous generation (which, duh, also doesn't need a backlight).
At those tiny sizes (and in small quantities), the price difference is minimal but commonly available OLEDs and and LCDs often come in different sizes. So the choice may have been more about dimensions than display type.
I can recall back in the early 2000s, we had a wave of people wiring up little character LCD displays for hardware monitoring. They inevitably looked awful at angles, washed out backlight, etc.
The super-lucky people could get a VFD display, which was a million times more legible, but they tended to be spendy, fragile, and warm-running.
OLED gives you everything: cheap, pretty cool running, and easy to read.
I've been experimenting with one of the Digole OLED modules to recreate what I wanted 20 years ago. :) (super-easy to interface to USB and program-- we've come so far!)
Not the OP, but small OLED displays are pretty low cost, and they don't need backlight management because the display is emissive. Viewing angles are usually nicer too.
