So by get getting louder they mean there's less dynamic range (range from softest to loudest) with the narrowing favoring a louder new-average. This however makes sense, at least to me, if we look at how music is being consumed these days. A great deal of music is now consumed during noisy activities -- working out, taking the train to work, driving -- compared to the '50's though early '90's (before Walkman's become true commodity goods) when the majority of listening was done in relatively quite environments -- at home in your living room, at concerts.

Portable music listening, in louder environments, necessitates the increased 'loudness' in order for songs to be heard by the average person. Most people aren't buying $100-500 IEHP/OEHP to listen to their music, so it becomes pointless to add in quieter and more subtle bits to pop songs since they are lost to the environment for many people. Additionally, radio broadcasts lose dynamic range during to transmission and most cars lack the audio horsepower (so to speak) to properly replicate tracks from local-media sources let alone from a radio broadcast.