Before OpenPOWER, IBM Power servers used a PowerPC-based don't-call-it-a-BMC. Either IBM decided not to release that chip to the outside or maybe Google didn't want it.
All the existing embedded PPC cores are targeted at network or storage applications; emulating a terminal / console / graphics card needs a bit of a different arrangement (at least if you want it to be compatible with some existing stuff.)
It does, it is used for soft realtime related stuff (power, fan, errorlog, etc). Throwing BMC related activities in there is certainly doable, but it would complicated things to the point where another chip would be tempting.
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.
I imagine Google was already using ASPEED BMCs in their x86 servers so they kept using it in OpenPOWER. There are some really specific things like PECI and VGA redirection that x86 BMCs need and AFAIK nothing besides ASPEED has those features.
I doubt IBM gave Google any thought in this matter. I'm pretty confident that it has a lot more to do with the way IBM does their market segmentation, and how that is related to hardware + software bundling and firmware. The ARM BMC fits in a HMC shaped hole.