Because that pattern forms a scale that allows for playing several intervals such that the harmonics line up nicely, so that they're either very similar or very

Because that pattern forms a scale that allows for playing several intervals such that the harmonics line up nicely, so that they're either very similar or very distinct, but not annoyingly almost similar. William Sethares has a good overview here: https://sethares.engr.wisc.edu/consemi.html https://sethares.engr.wisc.edu/consemi.html Note how figure 3 shows dissonance minima close to degrees 3, 4, 5, and 6 of the major scale when played against degree 1. Although there are many other possible combinations of notes, combinations with degree 1 are very important because degree 1 provides a kind of "home" that the music often returns to (the "tonic"). But the goal isn't just to maximize consonance, so these notes alone would make for a boring scale. For better musical value, we need some more dissonant intervals to build tension before it can be resolved with the consonant intervals. The obvious place to put them is in the big gaps. Putting degree 2 a single semitone above degree 1 is probably overdoing the dissonance a bit, but putting it two semitones higher gives a useful mild dissonance. Considering only combinations with degree 1, degree 7 could theoretically also go a single semitone above degree 6, which gives the mixolydian scale, but the major scale is more popular. I assume this is because of combinations that don't involve degree 1.