Exotic Chords

Hello all.  So I was looking at my dulcimer chord book, on the pages for four-equidistant strings, and noticed that there weren't many exotic chords available.  And I'm just wondering why that is.  I sadly have no head for music theory, but was wondering if it mostly because of the difficulty involved in stretching one's fingers all over the place on the dulcimer, or is it the nature of a diatonic beast?