Why redbook over SACD?

I believe it has more to do with the transport mechanism. All things being equal a purpose built mechanism that is limited to Redbook only can potentially do a better job playing Redbook CDs than a SACD capable mechanism can do when playing Redbook CDs.

The issue might also be a response to the market.  Redbook CD has rebounded to respectable sales levels

That’s just completely untrue — CD sales are down 97% from their 2000 peak and are bumping along at 33 million units comparable to what they were in the late 80s, and even vinyl is far outselling CDs at this point.  From RIAA data…
 

Physical sales dipped in the first half of the year, with vinyl revenues down 1% and CD revenues down a sizable 22.3%, so that physical revenues at large slipped 5.9%. Vinyl now significantly outsells CD, with vinyl shifting nearly twice as many units and generating more than four times the revenue. Total vinyl revenues for January to June were $456.9 million, while CD revenues were $108.1 million.

Remember DVD Audio Lol?!!!  Happy listening.   

I just obtained some SACDs from Tower Records in Japan.  Early 1970s recordings  featuring conductor Kurt Sanderling and the Berlin Symphony in symphonies of Sibelius and Shostakovich.  Remixed in analog, then converted to DSD.  I have been streaming the same recordings from what is presumably the high res Redbook layer on the same SACDs.  Both are excellent but the DSD is more detailed.  There is more sweetness to the strings, brass has more oomph, the double basses grounding the sound floor with more solidity.

  I have hundreds of SACDs.  There really is no comparison.  Redbook has improved and vinyl will always have its adherents.

  Sony made DACD difficult as others have noted.  It also became dragged down in a format war and was tied to multichannel.  Most consumers couldn’t be bothered.  Most thought mp3 through 5 dollar Apple buds was all they needed.  Convenience beats quality in our society 

@jazzman7 

There are two separate lasers in the same SACD/CD player/transport.  One for SACD, and another one for CD.  There are also separate circuits attached to those lasers.  One circuit does not effect another, so, I do not believe CD quality is affected by a SACD/CD player or transport.  For example, if the SACD laser stops working, the CD laser will continue to work and play redbook CDs.  The laser assemblies - optical pickup, sled, motors - and the module mounting which is all mechanical functions are packaged into a separate module called a "Traverse Unit".   This traverse unit may or may not be made by the vendor that is making the actual SACD/CD transport/player.  For example, Denon is a major supplier of this Traverse unit.