There are conservation reasons, which I don't have the scientific training to discuss, but I'd like to question your assumption that it is the "world's most fascinating monument". Yes, it is a large and spectacular tomb that probably has a lot of marquee artifacts inside, but those kinds of sites are not always the best to answer interesting research questions. Take for example the archaeological site of Gordion in Turkey, which has been under continuous excavation for 1950. Compared to the likely contents of the Mausoleum of the Qin Emperor it has for the most part been entirely unspectacular with the exception of the large golden burial in Tumulus MM and a few nice artworks. But as a research site it is one of the most important in the entire Middle East, on the level of Bogzakoy, Assur, Warka, Ur and other major sites. It represents one of the longest continuous human habitations known in Anatolia, was the capital city of the Phyrgian state(MM stands for "Midas Mound") and as such has some of the most important Iron Age monumental architecture of Anatolia, important evidence of the Hittite presence in central Anatolia, a notable Hellenistic town that can answer a lot of questions along with other Hellenistic sites about the Greek presence in Anatolia, a lot of plant remains that can tell us about the ecological history and food production of the region, and is generally nearly unparalleled as a laboratory for the archaeology of the ancient Near East. It may not be an enormous mound burying a famous Chinese emperor, but from certain perspectives a site like Gordion(and I pick that only because I know the archaeology of the Near East better than the archaeology of China) that preserves evidence about a wide range of human activities and habitations over a very long period of time is far more valuable as historical evidence.

EDIT: And I have not even touched on the humbler settlement archaeology, which for the most part surveys and excavates sites that barely make the front pages but can tell us things about daily life and historical geography that even the most impressive urban monumental site simply cannot.

It's still in the process of being excavated, but some of the discoveries we've made are already very interesting. The terracotta warriors are one of them. However, when they first opened the room containing the warriors the fresh air caused the paint on the warriors to flake off in a matter of minutes. So now they're being very very careful with the excavation, to prevent such a thing happening again.

According to the texts displayed on the site itself (not really rigorous historical material, of course, but presumably written in consultation with the archaeologists working on the site) the reason is archaeologists don't feel they're able to properly excavate it using what is currently available technologically.

According to legend, Shi Huangdi was buried in an enormous replica of the lands he governed using mercury to model the rivers and lakes of his empire. Preliminary soil readings have shown that there indeed seems to be a staggeringly high amount of mercury in the soil surrounding the probable location of his tomb. The feeling is that with the current state of technology it's not feasible to dig up something that is surrounded by so much mercury, both because it's impossible to guarantee the safety of those doing the digging and because it's impossible to guarantee the tomb itself won't be damaged when all that mercury is disturbed.

Since there still is a vast amount of work to be done on the terracotta army itself (which really is just the outpost of the tomb) and, but I'm conjecturing here, based on where the site is located and what surrounds it*, there is no real hurry in getting the thing excavated in its entirety, they've decided to leave it for now.

• The terracotta army site is located about an hour and a half by bus from the nearest major city, Xi'an, in an area that is mostly agricultural. Because of this there's much less of a hurry to excavate it, as there is little reason to suspect the city will be encroaching upon it anytime soon. This makes it a rather different site than, say the Ming Tombs (to which the outskirts of Beijing are edging closer every year) or the Jinsha site (which these days is well within the Chengdu urban area)