C2D is (4 off the top of my mind):
1.)Lower in TDP wattage consumption.
2.)Depending on the model, you can have up to 4MB L2 cach that is shared between the 2 cores.
3.)Because of it's microarchitecture, it does more per processing cycle, which out performs its predecessors (P4 family) at lower speeds.
4.)Since its based on the 65nm size, it should be cheaper to produce.