It's their need to line up a secure source of oil, which they need if they are to continue their drive to modernize.
China needs a secure, reliable source of oil.
The government of Sudan cannot afford to pay all the troops it needs to press its war against the South. It also needs the arms and ammunition which China can provide. Hence, they cannot afford or tolerate criticism of Sudan. Also Congressmen Wolfe and Tancredo, and perhaps Donald Payne and Chris Smith. I'm a 20 year global banking veteran, so the research is solid.