They couldn't without limiting the CD playback capability. DVD players take longer to recognise different kinds of discs, tend to be noisier, slower to respond, less reliable and don't perform to as high-standards as a CD player would with normal audio CDs.
Part of the beauty in the BeoSound 9000 for me, is it's simplicity, no extra frills other than what's needed and the original idea behind it as a 6-CD player which does a better job than any other on the market.