Thats the question China needs to answer, every time they buy or hold onto the bonds it indicates they are not confident about their own economy. If China was so confident they would invest the surplus back into their own country.
My another view of thinking is that China looks to inherit the western system instead of destroying it, they want to replace it with their own power. Thats the reason why they are being patient. But in the end it might be too late and the system itself get sabotaged by western block.