A: Good question! One credible answer we’ve heard has the reason dating back to the ancient age of vinyl records. In those days, to optimize the bass response recorded into the grooves on an LP, it was necessary to center the low-frequency sources in the middle of the stereo field so that the turntable’s stylus would track properly. Too much low-end on one side could create physical problems in the stylus tracking, so engineers centered the kick drum and bass guitar for best response on the average turntable. This simply became the accepted way to mix records, even after analog LPs were largely replaced by CDs and other media, and remains the standard today.
Another reason could be that as stereo became prevalent, it was important to ensure that there was solid low end available from either speaker, even when the listener wasn’t in the ideal sweet spot.