Intel owns the rights to the Pentium III CPU variant that resides in the Xbox, and Nvidia owns the rights to the Xbox GPU and the audio chip that encodes Dolby Digital on the fly. Without ownership of these key components, Microsoft had to provide backward compatibility the hard way--software emulation.
To make the situation even more difficult, the Xbox 360 hardware has practically nothing in common with the original Xbox. Microsoft replaced the original Intel CPU with a custom-made triple core IBM Xenon processor, and the Nvidia GPU was replaced with the 48 pipeline ATI Xenos GPU.
Ben 10 Omniverse wasn't the first Ben 10 videogame, but it was a landmark title for actually making a decent game of a children's license.