Comment by chrisweekly

On a guitar in standard tuning (EADGBE), the Em chord can be played by fretting only two strings (the A and D), both on the 2nd fret. It's basically the easiest chord to learn to play. And if you start in Em, it pairs with G, Am, C and D -- all of which are also very simple fingerings. Correlation isn't causation, but I'd bet dollars to doughnuts this is a factor, given the absolute dominance of the guitar in popular music songwriting.