It's not really in the U.S. best interest to push for unification. A unified Korea makes justifying a large military presence in Korea very difficult especially with large U.S. forces about 1200km south on Okinawa and 500km east on mainland Japan.

The situation in DPRK is a key piece of leverage in our AP policy and without it we lose some amount of influence in the at a time when the US and CN are vying for the region's economic alliances. Given China's recent run-ins over the South China Sea Islands I would also stand to reason that Japan is also not that keen on losing a large U.S. force in Korea either.

These issues will likely only amplify over time as China continues to grow both as an economic and military power -- the U.S. will need to continue to strengthen it's presence in the region and a large part of that is flexing its military clout.