Most traditional bluegrass guitar players play 14-fret dreads because the original bluegrass guitar players played 14-fret dreads, because bluegrass got started in the late '40s and early '50s, and there weren't any 12-fret dreads being made at that time - by Martin, anyway, and a bluegrass guitarist always played a Martin, in that time period. (Run-on sentence? Me?! You jest, of course!)

As a matter of sheer logic, a 12-fret dread would make more sense for traditional bluegrass guitar, which is just bass-chord rhythm with a lot of base runs. But custom and tradition rule in bluegrass, not logic.

Modern bluegrass guitar players often take lead breaks, so they use the whole neck, and need the extra frets.

So both groups use the 14-fretters, for two different reasons.

I think.

Peter.