I guess that one of the main reasons that they chose PPC was that Amigas were already going that way - PPC apps for Amigas already existed, providing a better software base, and making it more familiar to existing Amigans (which is who it's aimed at).
As for why they decided to design their own motherboard - that's a less obvious desision. For one thing, piracy would kill off OS4, so it had to be protected somehow. I personally agree with ruffneck and Smartalec about custom designs, though.
One thing is certain: From what i've read (and tried to understand!) in the features PDF, the entire kernel basically floats on the HAL - if that is ported then the whole OS can go with it, so it would be easy to port to x86.
