or maybe, they just wanted to make it hard to play fiddle tunes on guitars. Perhaps they didn't like fiddle music!

John's detailed explanation makes a lot of sense. It is however not difficult to play complete triads on a mandolin, which of course is tuned in fifths. The standard bluegrass chord forms include all three notes. Unless of course you want to get fussy and have them in order of 1st, 3rd, 5th bass > treble, but the concept of 'bass' is of course irrelevant to a mandolin.

Having owned a few longer-scale instruments tuned in fifths, like an oud, (well I tuned it in fifths anyway) I can testify that they are quite quite difficult to play those same chords on though.