I think it's because the top 100 or whatever chart you're looking at is meaningless these days. It used to be a fair representation of what people were listening to, I remember people taping the top 40 off the radio on a Sunday to listen to through the week, everyone was on the same page regarding new single releases.
Now we're atomized, I don't even know what the #1 single is on any given week and I don't care. I've got 30,000 tracks on my home server. There's no new artist who can speak for a generation like Dylan or Woody Guthrie could in their day.