We just don't listen to them anymore as they haven't stood the test of time. This was abundantly evident last weekend when I went through a stack of about 100 45s from the 60s and early 70s [...] I'd never heard of 90% of the bandsThere was a Cambrian explosion of music in that period, and virtually no one ever heard of 90%, probably 99%, of the bands. Countless groups sprang up all over the place and pressed records for their local markets. People have devoted careers to tracking down the recordings of that period; one of them, Greg Shaw (whom I knew for a while) had over a million records. He put out a series of influential compilation albums of his favorites that spawned an entire genre (garage rock). Decades later, this stuff is still being unearthed and released. There are entire series of albums devoted to the 1960s proto-punk of Oregon, or Denmark, or Uruguay. It's just amazing how much there is (enough that I'm skeptical of claims that more was recorded in the 80s), and much of it -- tastes vary, of course -- remains amazingly alive and good. A lot of people would disagree that it hasn't stood the test of time; every generation produces new acts in this lineage (e.g. Black Keys), and the underground history of the music continues to be handed down through the fanbase. Its popularity ebbs and flows in a 10-year cycle or so. Right now it's on an upswing.
Edit (by way of response to the rest of this thread, not to your comment): it's foolish to identify complexity with good music. Punk/new wave was a reaction against complicated, highly produced music which used an awful lot of chords. The entire career of bands like the Stooges and the Ramones was a self-conscious mining of the opposite aesthetic. Consider the famous 2-note guitar solo in the Buzzcocks' "Boredom": http://www.youtube.com/watch?v=QoYiQ8Qsozk&t=1m19s. The rest of the band was rolling on the studio floor laughing while the guitarist played it because nobody believed it was possible to do a guitar solo like that, let alone for that long. It's as far from complicated as you can get but still a great creation. Or think of John Cale's one-note piano drone that runs through the Stooges' "I Wanna Be Your Dog": https://www.youtube.com/watch?v=3gsWt7ey6bo#t=23. Cale came from La Monte Young and the Fluxus movement -- highly self-aware avant garde early 60s stuff that he put into a deliberately primitive incarnation. Not saying everyone should like it, but it's as influential and artistic as anything of the last 40 years.
The only rule is that you can't predict what form great art will take. The minute somebody thinks they've figured it out (number of chords? please), the muses jump and bless the opposite.