The original XBox was x86 largely because Microsoft had no clue what they were doing when they first designed the console. x86 is a dreadful architecture to work with outside of PCs, and even in the PC industry, x86 isn't really all that wonderful and is only in use because of how the PC market works. Apple knew if they stuck with PPC much longer on Macs they'd lose critical mass with software developers and they'd end up selling very expensive paperweights in a few years if they didn't do something.
As for game consoles: PPC has been demonstrated and proven time and time again to be technically superior to x86. To this day (April 2013) this is still holding true. Microsoft ditched x86 for PPC in the 360 when they realized basing the XBox on the x86 architecture was a horrible idea, both power and power-consumption-wise.
Alas, the next XBox and the PS4 are both going x86, whereas Nintendo is wisely staying PPC with the Wii U.