From my understanding, the reason it hasn't been opened yet is to preserve whatever's inside, not so much out of fear for mercury poisoning. For instance, the terracotta army was actually painted before it was excavated [1], but the paint curled and flaked off within minutes of opening the mausoleum, leaving us with a sea of drab stone statues. I think they're just waiting until they have the tech to properly preserve the interior.
[1] http://www.chinadaily.com.cn/china/2010-09/09/content_112783...