The knee-jerk answer to that question is because they always wanted us to own us, and this is certainly a reasonable argument. But we could default on our debts at any time, provided Congress has the will to do so (and they won't no matter how many Republicans are elected).
But then I realized something just now. China knows that most of our borrowing right now goes toward 'emergency' military spending with regards to foreign wars. In essence, they are lending us money to fight wars with other nations overseas. There are many benefits to doing this.
For one thing, if you have a nation that is hostile to you and your interests, what better way than to have another nation do your fighting for you. We certainly do this with regards to Israel and several Central and South American countries.
Another benefit is based on the premise that no nation can survive when it is in a constant state of war, especially when it is running a deficit. It is no real secret that China doesn't like our nation and what better way to destroy us than to support us in our constant wars.
Lastly, a nation that is constantly at war and has a large military apparatus will always be viewed with some level of distrust and often outright hostility by many other nations. Following the Civil War (or The War Between States), Canada was incredibly nervous about our large military presence that could easily be employed to go north. Granted, there was some inclinations that the Union victors would retaliate for British support of the Confederates, but those fears ultimately proved to be not justified.
So China wants to lend to us in order to facilitate wars they do not want to fight themselves, destroy our economic capacity, and ultimately tarnish our image with the rest of the world and make them more uncooperative.
This ultimately explains why we are have engaged specific enemies and not others, given the problems our nation still faces. When you consider what countries the “Axis of Evil” were (North Korea, Iran, Iraq), we’ve only invaded and conquered one so far. North Korea is China’s vicious little bulldog that they keep around to do all the things they can’t do in the global political arena.
As for Iran, they are a major contributor of oil right now and any war that we engage in with them would disrupt the oil flow. This would be bad for China, as they have a growing need for gasoline in their major cities.
So really, the argument could easily be made that China will direct our foreign policy through it’s lending. Any people who want to see foreign enemies dealt with militarily should keep in mind that our national interests will be subverted by other sovereign nations as long as we continue to borrow in order to bomb.