The standard guitar tuning is pretty much a direct descendant of the oldest lute tunings: even in the late 15th c., Tinctoris described a lute as being tuned ( relatively) c/f/a/d/g. So, in fourths with a third in the middle. As more strings were added, that went to G/c/f/a/d/g. The 4 course Renaissance guitar , circa 1560, was tuned similarly: c/f/a/d : again, fourths with a third in the middle. In the 17th c., the neck was lengthened and more strings were added: A/d/g/b/e. Then in the 18th. c., the sixth string: E/A/d/g/b/e. Which is where it is today. A bunch of fourths with a third in the middle.

Why? I m not a guitarist, but I think some of it would have to be the problem of scaling. To work well, an instrument string needs to be under enough tension to efficiently turn mechanical energy ( from the finger) into vibration, but not so much that it snaps. Some string materials are more elastic than others, and have a wider range of tension in which they'll work pretty well. Metal strings, especially the high-carbon steel ones of today, have a wider tension range than the gut with which guitars and lutes were strung until the 19th c.. All the strings on the lute/guitar are the same length. Ideally, you'd want them all tuned to the same pitch, but of course that would be less useful....fourths were probably the compromise: fifths would have compromised the bottom strings too much ( or broken the top ones) , thirds maybe not given enough range. I am not sure of this, but there were instruments that did have varied tunings- often open, more diatonic ones, that favored certain keys: citterns, both the 17th c. Brescian ones by makers like Virchi, and the later "English guitars" of the mid 18th c. . And those did have metal strings that could be more variously pitched. The open tunings made them more accessible to those who wanted simple chords for backing up a song. Brescian citterns could be found hanging in barbershops, in the 17th c. , so somebody could strum and sing while waiting his turn, and the English guitars are often to be found in portraits of elegant women of the later 18th c., like the stunning Dorothy Briggs of Virginia, whose portrait is at Winterthur.

The cittern actually is also the ancestor of the Appalachian dulcimer: another instrument with fairly simple chords, easy to sing with.