Originally Posted by Paratrooper1n0
Because the Xbox and Xbox 360 use the same line up of CPU's.
PS2 and PS3 use different chips.
Uh what? The Xbox uses a Pentium III while the Xbox 360 uses 3 IBM PowerPC cores derived from the Cell Broadband Engine's PPU. The Xbox uses an x86 architecture while the 360 uses PowerPC which is a RISC architecture. Backwards compatibility is achieved through emulation since the 360 is powerful enough for that.
PS2 emulation should be possible on the Cell and RSX though developing the emulator can be quite difficult. Architecture-wise the SPUs can be used for processing instructions for the 2 vector processors with the PPU handling CPU instructions. I don't know how the Graphics Synthesizer would be emulated though.