Why redbook over SACD?

My guess is that it is a Sony copywrite issue. There is a fee to pay, and transports have an issue that they cannot just output the DSD layer through a standard Digital out connection without violating copywrite.  So, the vendors that attempt it on transports use a "proprietary" connecton to the DAC.  For example, I have a McIntosh SACD transport that connects to a McIntosh DAC with a proprietary DIN connection that is supplied with the Transport.  The other digital out connections on the McIntosh transport only output CD redbook.   Most vendors just choose to include a DAC within the SACD/CD player to solve the issue.   An internal DAC within the SACD/CD player does not violate the Sony copywrite.   The other option is to rip your SACD to DSD - dsf files and then you can feed them to any DAC you choose. There is a simple process to rip DSD off SACD using old Sony blu-ray players that you can get super cheap on ebay, ie like a BDP-5100S. There are a few commands you put on a USB stick and the player will download the DSD layer to the USB stick. 

The issue might also be a response to the market.  Redbook CD has rebounded to respectable sales levels whereas SACD seems to be an ever shrinking format.  Clearly Sony should have opened up this format when it was first introduced.  It is a shame, but it is what it is.  Technically too, the early limitations of Redbook seem to have been largely overcome and this fact has definitely given the format reconsideration.  If anyone from Sony is listening in, it might not be too late as there seems to be a demand in the market for premium sound quality.  HIREZ does not fill it.  Vinyl can, but to do so requires extraordinary dedication and expense that only the most dedicated are willing to undertake.  Redbook CD reaches, but doesn’t quite scratch the itch even at it’s current best.  So, SACD struggles on, but it is so very limited in scope at this time that it just is not what it could be or should be.  Sony, is anybody there?  Are you listening to us?

cos SACD adds nothing. if Redbook is dereived from a master that is excellent, SACD is just a needless pain

everyone is right!

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.