It wasn't a POWER thing since POWER has been superscalar since the 604 back in '94. This was because when you're designing for a console and paying by the wafer instead of shopping around for a pre-finished unit one needs to consider die area more strictly. Something had to give in the design and given MS controlled the design of the whole stack they thought they could do parallelism at the thread level in the OS scheduler rather than having to devote massive amounts of die area to it.
