What I notice is that the common open guitar tunings are usually in major. Of course anyone can tune their guitar any way they want, but major seems to be the standard. I was wondering why. For example let's take Open-D D–A–D–F♯–A–D. How would you play a minor chord on that? Whereas if you'd change it to a minor chord by changing F# to F, you can easily play a major chord by adding just one finger to the bar. I suppose there must be some benefits about using major tunings then. I'm just curious to understand.
I think the main reason is simply selection bias. Open-tuned guitars are most often used in blues and genres derived from it, and blues traditionally uses almostly exclusively major chords (usually with added sevenths). No minor chords – you do find the minor-chord notes, but in the melody, as blue notes, and those can very well be played on a major-open-tuning guitar, especially with a slide.
It's probably not coincidence though. Major open tunings have a big advantage (in particular historically speaking, pre-electronic-tuners) of being very easy to tune by ear, in just intonation: both for the perfect fifths and major thirds/tenths over a low bass it's easy to hear the beats against their exact 3:2 and 5:2 ratios, which makes re-tuning a guitar between songs much quicker. For minor tunings, this is significantly more fiddly: the 12:5 minor tenth can hardly be used for accurate tuning, you instead need to tune the fifths first and then tune the thirds as 5:4 down from them.
Even if you do get a guitar tuned to a just-intonation minor chord, this is actually problematic because the minor-note string will be a bit sharp compared to it's 12-edo value. As a result, if you finger this string to a major third, it will be even more sharp than a 12-edo major third, and won't sound very nice at all. So you need to make a compromise: either you tune the minor-third string flatter than JI, which takes away much of the particular charme of an open tuning, or you just won't get usable major thirds at all. (Unless you have a guitar with e.g. 19-edo fretboard.)
By contrast: if the guitar is tuned to a JI major chord, any fingered note on the major-third string will be slightly flat, but this is easily compensated by bending it up slightly. Not possible the other way. (Well, again with a slide it could be done, but that won't work if you use the slide for bar-ing a chord and a finger for changing the minor note into a major.)
Tunings like DAdgad circumvent the issue entirely, by avoiding thirds in the open strings altogether. The fifths and fourths are all easy to tune by ear, and they're all compatible with 12-edo.
First, let’s talk about playing minor chords in major open tunings.
For example let's take Open-D D–A–D–F♯–A–D. How would you play a minor chord on that?
Fret the second fret on both A strings and it becomes a B minor chord. Use a barre and two fingers as appropriate to play any other minor chord. And that’s just the first way to play minor shapes I found in under a minute.
Also, if you’re in a band, any major chord on guitar can be turned into a minor seventh chord by simply having the bass player play a bass note a minor third below the root of the major chord. For example, a bass playing a low B makes D-A-D-F#-A-D sound like a B minor seventh chord.
But open tunings were very popular with solo delta blues musicians, and they wouldn’t have had that option.
In blues and rock, open tunings are most often associated with slide playing. The popular major open tunings allow the playing of both major chords and open fifths (power chords) with the slide. And I think that leads closest to a good answer:
In delta blues and blues based rock, the chords are usually major chords or power chords, even when the melody in the vocals uses the lowered third scale degree. In short, the chords may be major while the melody is minor. That’s an over simplification, but is a decent approximation.
Delta blues played with a slide often ends with a slide up to a major chord or a dominant seventh chord, which is another reason to prefer an open major tuning over an open minor tuning.
If you’re like me, you’re probably wondering if the tuning came first and that informed the style and tropes of delta blues and the rock it inspired. I haven’t been able to find an antecedent for delta blues open guitar tunings, but I do know that delta blues guitar players most often had to tune 100% by ear. The made their guitar fit their vocal range and tuned up without anything other than their ears and voices. I suspect (edit: confirmed by leftaroundabout’s excellent answer) it’s much easier to tune a major open tuning by ear.
Once a body of blues and folk music was created in open major tunings, it motivated later musicians to use those same tunings to expand the literature.
I looked to find a deeper history of open major tunings and haven’t been able to trace it back before the 1920s, but I wouldn’t be surprised if there are folk tunings and tunings of other instruments that led to modern open major guitar tunings. 5-string banjo is one example of a related instrument with an open major tuning.
The minor chords and scales are a kind of circumstantial sound. They sound "sad" because they do not really belong in nature-- there's something off about them.
The harmonic series are the notes which a string naturally vibrates on, and are equal divisions of the entire string. For E:
E, E, B, E, G#, B, D (detuned). There's no G to be found-- the minor third is not found in the natural vibrations of a single string.
That being said, I don't believe it IS that rare to have a minor open tuning. You could tune your strings to (from the bottom): E, G, E, G, B, E.