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I know that tuners are one of the features that distinguish a classical guitar from an acoustic one.
But why are the tuners the way they are?
And are they still this way in this day and age because they just work or because they're perfect for their purpose?
EDIT: I meant the tuning pegs.
When what we think of as the classical guitar was normalized, it was just after luthiers switched from friction pegs like you would see on violins to the geared open-backed pegs, which were the advanced cool new tech at the time. They hit an optimized form early, and couldn't make it better without making an large departure, like steel strings with higher tension, so the guitars that get made are made to be the best and prettiest versions of that.
You can find acoustic guitars with that kind of in-line open-back tuner. I think I own four. But the closed-back style you often see are lubricated for smooth operation. I would guess the post is made from higher-quality metal to keep from being bent from the higher tension, but that's just a guess. There are also locking tuners, but I normally see them on electric guitars with whammy bars, not steel-string acoustics.
I could imagine classical guitars with the more modern tuner styles, as I have seen violins with geared guitar-style tuners and with geared tuners built to look like friction pegs, but musicians often go with their eyes not their ears, and would be likely to avoid guitars that don't look like the ones their heroes played.
It's because the strings are different.
If you look at the shafts that strings are wrapped around, on classical guitar they are plastic and have much larger diameter than on acoustic. Nylon strings have lower tension than steel ones, and stretch much, much more when pulled into tension. The shaft has to be larger so that more string can be wrapped around it, and plastic, because metal would be heavier and nylon strings don't cut into the tuner like steel ones. It would be possible to build a non-slotted headstock with tuning posts with similar dimensions, but it would look ridiculous, and because of the huge number of string wraps, string break angle at nut would be inconsistent.
In other direction, acoustic and electric guitars with slotted headstocks do exist. The tuning shafts are steel instead of plastic and the gear ratio is typically lower than on classical guitars. They are uncommon because the slotted design makes string changes a huge hassle and the design doesn't have that many benefits.
The main reason is that nylon strings need to be stretched much more than metal strings to raise the pitch by the same amount. For this reason classical guitars need thicker pegs with larger circumference, allowing to reach the desired pitch faster, with less rotations. Metal string guitars need thinner pegs for better control of the pitch while tuning.
Some other differences might be historical, but there also could be some other technical or musical reasons. I don't know.
The tuners technology is being developed e.g. you can find classical guitar tuners with bearings where the pegs rotate in the holes, and with other improvements.
