Because China holds almost all the leverage here. Which means one-on-one talks would be all carrot and no stick for NK, which is exactly what Kim wants.
The 5 party talks are effective because when the Nork's break their word , we're not the only one with egg on their face.
China's reaction was reported as the strongest condemnation ever. They rioted in S.Korea. Abe's election in Japan was a good sign. Abe talks of changing the 'pacifist' constitution of Japan.
Let's roll!!