The games single player is basically a simple premise of gangs. It was like an expansion of GTA2.
You wipe out gangs and eventually there's the gang bosses and a final boss. You can, for example, fight the top dude the second you find him.
Thing is... in a destructible environment you'd be able to take him down right away without needing to level your stuff because you could just take down the building he's in.
That's why there's this separation. The single player would be broken with destruction.
MP mode is 10 players
plus your playing a char who is trying to save the city not destroy it