Simple answer- China. China rules the Far East. It does not do China any good to have a conflict again on the Korean Peninsula. Second best answer is Wal-Mart. Believe it or not China needs Wal-Mart. A large part of many cities in China depend on Wal-Mart ships loading up on China goods.
Monitor: Clashes near Damascus despite truce there, five killed in Aleppo (Reuters)
from the article: Lieberman: How will the world stop Iran if it can't stop N. Korea?