DEN isn't a massive UA hub -- it's in the middle of the pack at best -- and there isn't much O&D traffic from Denver to China. UA's focus in DEN is domestic, and the international routes they do have from DEN are mostly to cities where they have joint venture partners to help generate additional traffic.

None of that matters, though, because UA can't simply announce that they're going to begin a flight from DEN to PEK. The US/PRC bilateral aviation treaty is extremely specific and lays out the city pairs that may be served by airlines from each country. DEN isn't on the list; therefore, UA can't fly from DEN to China.