Multichannel SACD is 5.1 by intention. Sony/Philips have specified it as 5.1 in order to keep compatability with home theater specs. This is obviously a business decision. Apart from algorithms like bass management that are tied to specific definitions of the channels, I know of no reason why other multichannel surround formats (e.g. 6.0, 2+2+2, ambisonics, etc) could not be coded as SACD, but S/Ph have not as yet condoned it.