I guess it is a question of musical style - mostly. I disagree with the thought that compression is at fault: 320 compressed music can sound excellent - and agree totally with the lodness part.
If you recall, a few years back, a fad popped up, called "lo-fi", attempting to keep things "simple and back to basics". The result was terrible: bands like Blur (undoubtedly, sucsessful) put out a CD containing some excellent music (and some not so good), that sound wise is terrible (and one can only hope they'll wise up one day, and remix the whole album).
Heavy Metal bands - unlike in the past, exagerate drums and based,just to cover the lack of any music. Just for comparacent, you should listen to AC/DC's "Back in black", which is not a pinnacle of audio, but still sounds miles ahead of anything produced today, and although heavy metal isn't my cup of tea, it is one of my favourite albums of all times.
On the other hand, some Rap CD's are a good surprise: I guess that the fact that Rap is basically very minimal, more attention is put into each and every sound.
I would add another factor in: The mass market does not listen to music like we, AudioKarmans do. Us, we could listen to whole albums as they were when they were vinyl, and 40 minutes long, with a theme. The current average CD consumer listens to 3 minute songs, and jumps to the next song even before the 3 minutes are up. Short attention span and remote controls are to blame.