The difference with CDRs is that you can burn and sell as many as you like and the MCPS don't know how many you've done, so there's no proof of what royalty payments you owe the artist.

In contrast, a CD pressing run (unless it's a short-run test pressing) has to be registered with the MCPS.

Nothing to do with audio quality or CDR longevity, though that might be an issue too.

Anahata