Time makes a lot more sense than stops: for example, at Columbus Circle in NYC, a downtown A train that's one stop away is about 3.5 miles out. A downtown 1 train that's one stop away is less than a half mile out. Except if the downtown A train is running local, in which case it's then about a half mile out.

Time, on the other hand, can be meaningfully displayed to a rider no matter what information they already know. (It's also fairly accurate in practice in my experience.)