Gives you a sense of the relative sense of time between us and the East. Our guys would probably want to have at it so the tomb could be explored/catalogued in our lifetime. The Chinese may be content to wait for centuries.
I suspect another reason may count aswell.
I heard there is a prediction going round in china that when the tomb is opened it will be immense bad luck.
Mao may be anti-religion but maybe not completely untouched by the idea of the 'curse of the first emperor'.
Also if they opened the tomb and anything goes wrong it would be immense disgrace on Mao's government. It ain't to populair right now to begin with.