If you just look at the shapes of the diatonic triads on each set of 3 adjacent strings, you can see a good reason. On the group of 3 smallest strings, and the

If you just look at the shapes of the diatonic triads on each set of 3 adjacent strings, you can see a good reason. On the group of 3 smallest strings, and the next group over, the shapes are all very reachable by the hand. On the thicker string groups, the shapes require a wider spread of the hand — especially the half-diminished triad. The smaller, more comfortable shapes on the thinner strings are made possible by the kink between the G and B strings.