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.
Gee, if Mao's involved this is really getting scary. He's been dead since Sept, 9, 1976.