Older types of dulcimers were usually played in the traditional (non-chordal) style, and if the fretboard was in the middle of the top of the instrument, a TALL fretboard made it easier to play with a noter in hand. Rather than a tall fretboard being of solid wood (possibly lessening resonance), making it hollow allowed for more sound vibration and resonance. In effect, a large tall hollow fretboard with its own sound holes was like having a very narrow scheitholt sitting on top of the larger body. In fact, there are some old examples of mtn dulcimers that very much resemble a narrow schietholt built with a larger sound chamber body underneath it. It's one of several possible paths of evolution or 'missing links' in the history of very early dulcimers and zitters in American. More than one of those 'dulicmer evolution' paths may have occurred long ago in different regions.
As modern times approached, people began to play dulcimers more often in chording style, fretting all strings and making chords with the fingers, adding double melody strings- this all encouraged wider fretboards and the need for a tall fretboard that accommodated a noter was much less. Dulcimer bodies became larger and fretboards became wider and less tall...and hollow tall fretboards became less needed for resonance or for noter comfort.
Those irritated by grain of sand best avoid beach.
-Strumelia proverb c.1990
updated by @strumelia: 11/08/16 10:08:12AM