Actually, Sony and Nintendo were also on POWER at this stage. Sony ran a particularly exotic variant, the "Cell" processor; this used a single POWER core that was tied in a ring architecture to several specialized coprocessors.
For the next generation of consoles, AMD's integration of ATI graphics onto a single die CPU/GPU/APU proved irresistible, and both Sony and Xbox switched to AMD x86 variants. The Nintendo Switch adopted the Nvidia Tegra ARM architecure.
IBM essentially had one generation of consoles. I don't know why they didn't fight harder for this market.
