| Because if you put electric bass strings on an acoustic guitar with its scale of around 24" to 25", there won't be enough tension on the top to make it move any air. An acoustic guitar (or viol family instrument too) is simply an air pump. The strings make the top vibrate, and the vibrating top moves air. To keep the instrument from falling apart it's got to be braced. So the strings have to have enough tension on them to make the top move. That's why most acoustic guitars sound "better" with heavier strings- and why light acoustic strings are a 12-53 and a light electric set is 9-42.
Add in what's been said above about having a body big enough to move low notes, and that's why viable acoustic bass guitars are so very rare. IME only three count- the old Ernie Ball Earthwood, the Guild B-50, and just maybe the Tacoma Thunderchief. Note that not one of them is in production, probably because the market for a TRUE ABG is severely limited and skewed to looking for cheap instruments- which won't happen with a GOOD acoustic instrument.
That's why there's a plethora of crappy sounding small-bodied guitars with longer necks and bass strings.
John
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Lakland Owners' Club # 248
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