Simple. They wanted to make up for the lack of amazing graphics, Euphoria and DMM by adding a duel mode and extra missions. Extra missions are available for the PS2 version as well, and it also has new Ragdoll physics. The DS version I don't know much about besides the fact that it too has multiplayer. As for the PSP version, it has a huge selection of custom maps for it's lack of pretty much everything.
They tried to balance everything for every system so that no matter which one you buy, you get something the others didn't. (I guess it's like Soul Calibur)
I think the sad part is, during my entire time playing the Demo, there was less destruction of the environment then there is in the Wii video