A lot of it has to do with how music is mastered nowadays. Frankly, I think music production was at its peak in the 1970s, when analog technology was being perfected, the loudness war didn't really affect albums, and nobody was trying to shoehorn everything into digital. In the 1980s, digital shoehorning began, and the loudness war started to affect studio albums rather than just singles, so sound quality started to drop. Digital recording improved vastly in the 1990s, but by then the loudness war really started picking up steam, and this is when we really started to see the beginning of "modern" sounding music.
This all being said, the 80s and 90s are actually two of my favorite decades for music. Why? The 80s had a large number of good underground (thrash metal, industrial) artists, and I actually like some of the New Wave stuff from that decade as well. As for the 90s, this is when alternative music hit it big, and during the first half especially, there was a lot of back-to-basics creativity, with artists eschewing over-the-top production.