This is the first important factor. The trouble is, the OP is comparing two different things: the best from the past, and all from the present. In the here and now, you get to hear a lot of stuff that will be forgotten within weeks, along with future classics. But for music more than about six months old, that filtering has started. The further back you go, the more dross has been filtered out, by memory and by radio play, and the higher the proportion of classics. This gives you an artificial idea of how good the music was in the past. As someone who lived through quite a few eras of music I have to tell you there has been a lot of excrement over the years, but most has been forgotten. It's exactly the same reason why people think that the past had better TV.