A: Because the United States has bases in South Korea.
If North and South Korea were reunified, American troops would likely have free access right up to their border. Air bases and listening posts may be established. This would bother any nation. Considering America’s aggressive forward deployment throughout asia, China cannot be expected to cede one of it’s few buffer states.
Some who are paying attention do point this out, the Council on Foreign Relations writes:
China’s support for Pyongyang ensures a friendly nation on its north-eastern border, and provides a buffer zone between China and democratic South Korea, which is home to around twenty-nine thousand U.S. troops and marines…. North Korea’s allegiance is important to Beijing as a bulwark against U.S. military dominance of the region
The East Asian Forum, also deems this motive worth a mention:
Taking into consideration the current balance of power, unification is likely to lead to absorption of the impoverished North by the rich South. For China it might mean the emergence of a stronger US ally ― or, at least, another ‘unruly democracy’ ― right on its border… it would prefer to maintain North Korea as a strategic buffer zone.
In both cases however this motive is pushed off to one side, with much greater weight given to China’s concerns about “instability” and the associated waves of refugees that might come flooding across its border, which is also no doubt an issue, but unlikely to be the sole basis for a long-term strategic partnership.
I feel they underplay it, for the same reason that it has been underplayed (indeed, unplayed) by the media during the reporting of the recent flare up.
No one, for example, suggests that if America could offer china a deal where if the latter brought Pyongyang into line, and into the global community, the former would dismantle it’s military presence in South Korea.
Such discussions would require the correct implication that for the last 60 years America has militarily dominated most of the globe, threatening others by doing so – and we mustn’t speak of such things.
Amongst almost the entire intellectual Anglophone world (even among many of those attacked from the right as being anti-American) this has somehow become both a taboo and a given. Something plainly and eminently obvious yet at the same time an unspeakable secret.
For those who have no idea what I’m talking about I suggest this link, in which professor Noam Chomsky Discusses America’s long term agenda of global dominance.