I'd say it comes to down recording for each of these. Pop music - music promoted over the radio is recorded with tons of compression and limiting to maximize sound for signal strength over radio. AND the frequency capacity of CD's is limited by the format - it doesn't go down as low as "full range speakers" ... so...you're going to hear more depth, more of the room...things that come from deep bass and sub bass. So...midrange focused systems will be pop-o-licious but their weaknesses will really be displayed by uncompressed unlimited recordings
Rock and Roll Snobbery
Can anyone explain why otherwise high end, musical systems might "not be good for rock and roll?" Or why a system that sounds fine for pop and rock might not do justice to classical and jazz? It seems to me that a great system should sound good with all types of music and that a good for classical system might be deficient in bass which is not exactly state of the art.