I think the main thing is the fact that they have so many people and since they were closed for so long, it's sort of like an consumer market that still has a lot of untapped $$$.
The US has to weigh whether that untapped consumer base is worth blowing off. Obviously, as of now, it's not, as we have many US companies invested there and we tread a fine line when saying something about their human rights or Taiwan. The US says something but that's about it. Nothing major like sanctions or anything like that.
If the US turns it's back on China, China will do all their business with the European Union or Russia. Not that the US will fall apart or anything like that, but it's bad business.
On a side note, I'm not buying the military analysis by the poster before me. Though they have a lot of people (North Korea does too but that doesn't mean anything) and nuclear capability and deteriorated Russian military advances, they are no where near our military capability. It would be a losing and stupid battle for China to fight us militarily. That said, it would be stupid for the US to do anything militarily against China for the fact that it will cut off the untapped market I was talking about in the beginning.