Is there a reason for this? What exactly is the reasoning behind it?
I'm not a luthier, but my understanding is that the 12th-fret join is used to maximize volume and tone. The 14th-fret join is a compromise for playability, and is more acceptable on a steel-string instrument, where there is already plenty of force on the bridge to drive the soundboard, due to the higher tension strings.
With the geometry associated with a 12th-fret join, "the bridge is safely away from your sound hole and well enough into the lower bout of the soundboard to get it well activated and moving effectively. ... The general understanding goes that the further the bridge moves down in to the more central ‘sweet spot’ of the lower bout, the more power and sustain an instrument will have." - quoted from a blog entry ("The question of the ‘sweet spot’ 12th fret join") on the Turnstone Guitar website.