I mean, the OCC and some of the other on-chip cores are based on the 400 series cores, but what I mean is there were/are tons of SoCs from NXP almost purpose built to serve as BMC-style control chips, with reasonably powerful cores and all the peripheral I/O it'd need. These were used (AFAIK) in Sun servers, so I wonder why not in OpenPOWER.

I'm guessing cost/availability is really what it boils down to, since the ASRock chips are probably just that much cheaper and better understood.