Seems like common sense; it's not the 70's anymore.
1) We can through technology make better gear that can process hotter signal without distortion.
2) Thanks to portability, it's more common that someone will be listening to your mix in a noisy environment, anyway.
3) People are now listening on a wide range of transducers: from artificial low end boosting (Bose) speakers to low end lacking ear buds.
4) The louder you have to turn your speaker, the more you are hearing the frequency response of the drivers, as opposed to the mix.
5) Generally speaking, people aren't buying whole albums anymore, they buy singles. Albums aren't mastered as a whole like they used to.
6) Personal preference. I don't care if it's taboo, I think trading the transient for more sustain is GENIUS. I LOVE compression.
7) Nobody wants to adjust the volume of iTunes every time they go from a modern heavy metal song, to an acoustic cover of it.
The radio isn't the main target anymore: It's mediums that don't add extra compression, like you-tube, spotify, last.fm & pandora.
9) Majority of Zep records were made with like 8 tracks total. You didn't need as much if any compression to glue together 6-8 tracks.
10) People like shit loud. Only audio engineers complain about loudness. Music fans and most musicians (especially rock) think it's awesome.
I could probably go on but I'm rocking out to some throwback Killswitch Engage as I type this. And it's REALLY loud. I love it. My neighbors probably hate it. If you don't turn it down. You'll still end up hearing more out of the track since the dynamic range is so selective.