> Why is a seven-note scale called "diatonic" anyway
The prefix of "diatonic" is "dia" ("through"), not "di" ("two"), but nobody really knows what exactly its origin (διάτονος, "diátonos") was supposed to mean[1]: it's probably either something like "through the tones" or "stretched out tones".
The second meaning is closer to the modern definition we have for diatonic: the 7 (out of 12) notes are selected to be as stretched out as possible in the octave, that is, each adjacent note is either 1 or 2 semitones apart, and the two 1-semitone-apart pairs are as far as possible from each other (note that the second requirement excludes e.g. the ascending melodic minor[2] from being considered diatonic, even though it has 7 notes).
The prefix of "diatonic" is "dia" ("through"), not "di" ("two"), but nobody really knows what exactly its origin (διάτονος, "diátonos") was supposed to mean[1]: it's probably either something like "through the tones" or "stretched out tones".
The second meaning is closer to the modern definition we have for diatonic: the 7 (out of 12) notes are selected to be as stretched out as possible in the octave, that is, each adjacent note is either 1 or 2 semitones apart, and the two 1-semitone-apart pairs are as far as possible from each other (note that the second requirement excludes e.g. the ascending melodic minor[2] from being considered diatonic, even though it has 7 notes).
[1] https://en.wiktionary.org/wiki/diatonic
[2] https://en.wikipedia.org/wiki/Jazz_minor_scale