I remember trying to explain a few basic musical principles to my brother many years ago, who was playing guitar and singing.
He trained as an engineer and then went into computers, software, he is still at the top of his profession in the US and working for Bloomberg.
His most often asked question was 'why'.
I don't think I ever got through to him that in music theory you could probably spend many years trying to find the 'why' of quite a lot of things, completely missing out on the whole purpose which was to learn the 'language' so that you could use it in practice. You may just as well ask why a carrot is called a carrot and not a cauliflower in many instances.
Major and minor. Well, composers have used both versions ( all versions of the minor ) for a very long time as well as various modal stuff, and if you just accept that the answer is 'because that's how it is' . It seems quite logical to me that the ( natural ) minor of each of the twelve major scales starts on the sixth note of that scale ( three semitones below the tonic ) and then uses the exact same notes as the major but the sound is 'sadder' due to the lowered third. That way anybody can work out chords, keys and be well on the way to understanding, possibly composing and also playing music.
If you take the example of C major and A minor. If you had D as the relative minor, you would get a minor sounding scale, but with a tone between notes 5 and 6. The standard harmonic minor would then need the 6th lowering and the 7th raising, the melodic minor would need a raised 7th ascending, then cancelling out on the descent and the 6th lowered. Yes it would work, but not within the current 'rules' Likewise if you start on E, you have a minor third, but the fact that the second note is a semitone sort of means you are still aurally in C major. B also does the same thing, doesn't work, sort of pulls towards C, probably because it is the leading note of C.