I've thought about this a fair bit myself, and I think it's a combination of a couple of the reasons stated. First, it's a reasonable range in which every othe

I've thought about this a fair bit myself, and I think it's a combination of a couple of the reasons stated. First, it's a reasonable range in which every other note can be played within the string before. But more importantly, I think it's just because it's incredibly easy to hear a fifth for tuning. Humans are fairly adept at hearing minute differences between two of the same frequency (i.e. a few hertz or cents off) when two notes are exactly the same, a fifth above share the most harmonics within that reasonable fingering range.