some musing:
the number of strings is constrained to allow for chord voicing across all the strings,
and to provide range,
open chords with minimal fingering complexity radically increase accessibility,
duplication is critical to support those,
capping with the double octave of the lowest note makes up-and-down strumming work,
if you're going to break pattern to coerce the highest string, it's extremely useful to also provide its dominant (5th) which makes sounding those top strings together just "work" (especially: with a bass note on the lowest string), with the side benefit that it allows learned scale patterns to work across those two strings, where most melodic playing will be up-an-down and third- fourth- or fifth- jumping or stopping,
...add all this up and it's a collection of mild but clear wins that together really strongly entail the current system.
Robert Fripp's 500 page book on guitar playing no doubt explains why this is Wrong and his preferred all-fourths tuning etc. are Better though. :)