My take... up to the late seventies a lot of mixes were very focussed to the mid range. During the 80s the 'hi-fi' thing became more popular and mixes started to reflect that, brighter, slightly scooped low mids, and with a more pronounced (but still controlled) bass. In the 00s the loudness quest has pushed the upper mids and tops even further forward. It's almost like the definition of 'flat' in a musical sense has changed. What was once a rich brown in tone is now a garish pink.