It's not about E major as such -- all the songs were transposed to C major/A minor scales. It's about chord III in major scale or chord V in minor scale. In classical music chord III is basically never used in compositions in major keys. In minor key music, chord V is typically altered to major as it leads to the tonic much better. Often, for instance, melody ends in G# and A. Minor V, on the other hand, is kind of ambiguous and rarely, if ever, used.

EDIT: Thinking about it, E minor is fairly common in phrygian scale (it's the tonic chord), however, phrygian, unlike dorian or myxolidian, which are used at least sometimes, is super rare -- its dominant chord, B dim, is dissonant, which kind of sucks.

Phrygian is quite common in metal, along with phrygian dominant. Great scales for dissonance and tension.

Functional harmony kind of goes out the window once you stray from the major/minor modes anyway. You have to lean heavily on the root to maintain the tonality, or else the perceived key center tends to shift.

Ah, I haven't been aware of usage of phrygian mode in metal music. Maybe that's what skews the ratio of Emaj/Emin towards the latter?

The author notes that the analysis is based on 1300 songs from the Billboard top 100 in the last 2 years.

I think it's safe to say the effect of metal songs using phrygian is negligible.

And the phrygian used in metal has modified major third (G# instead of G) which, again, results in E major, not E minor chord.

The minor V is used a lot in reggae and modally based music

This is written with all songs converted to the key of C though. So in guitar-friendly keys you're probably looking at the key of E and there the Em becomes G#m; or key of A and C#m.

Sure -- slap a capo on the 4th fret and there it is. :)

(Also, songs are often written in one key, then transposed in the studio.)

Oh, I wouldn't slap it on the fret ;)

There’s a key frequency chart and it makes sense that C and G are so popular. What didn’t make sense to me is why D and A aren’t more popular. Eb and F are somehow more popular. If this list is dominated by guitar, I’d expect D and A to be more popular since you can play I, IV, and V in those keys using open chords.

Eb is probably disproportionately popular due to the common practice of tuning guitars down a half step, making Eb instead of E the natural/easy root chord.

I'm not sure why F would be more popular than D and A. One hypothesis would be that it has fewer accidentals; it's not a fun key for guitars, but many contemporary pop songs aren't guitar-driven anyway.

Hypothesis time: the predisposition to flats is due to Big Band, Blues and Jazz music being predisposed to it. And there it is logical due to the B flat and E flat tuning of the horns (their C is our B flat or E flat).

Make life even easier with an Em7!

Em7 is easier to argue for. It's basically the dominant (G mojor) with added 6th.