Russia is a big place with limited internal transportation. Land routes from Russia can only supply a fraction of China's fossil fuel import requirements. Much of it still has to come via vulnerable merchant ships. China still lacks the capability to protect their sea lines of communication, although that might change in a few years.

The other thing to keep in mind is the Chinese, quite reasonably, don't have a lot of faith that the Russian government they make deals with and invest in now will be the same one honouring those deals and investments next decade. Russia has a lot of risk, a lot of problems and not a lot of cash.