My understanding was that they don't lack the technology, but they just don't want to excavate. During the '50s, the Wanli Emperor of the Ming Dynasty's tomb (Dingling) was successfully excavated. However, the political upheaval of the Cultural Revolution led to revolutionaries storming the museum housing the artifacts, and "denouncing" the remains of the Wanli Emperor and his Empresses before burning them. As a result, the PRC's policy after the end of the Cultural Revolution has been to not excavate anything except for rescue purposes.

You might be better off asking /r/archaeology something like this.

Personally, I think the problems are a/ having a robot bump into, say, a partially decayed silk scroll or already-humidity-compromised clay relic (they're not all that precise to begin with) and b/ the fact that you'd basically be sending something radio-controlled into a shielded area - mercury is a metal, and a conductive one at that. Super tricky.

I'd hate to be the guy who set Qin Shihuang's tomb on fire with a short-circuiting RC car.

But this is really a question for archaeologists.

Conservation concerns are the bigger issue here-you have a massive amount of archaeological material in highly uncertain condition and which probably requires more advanced conservation methods than we have currently.