This question has puzzled me for such a long time...
I've heard recordings from the 30s by early blues musicians such as Robert Johnson and, while it is still possible to hear the music, the quality is always poor. There is background noise, crackling from the original records the songs came from, and other interruptions.
However, Beethoven and Bach and other classical composers were around hundreds of years ago. When I hear any of their recordings, they're always near-perfect soundwise.
So what gives? Did they have MP3 players back in those days?