The gritty Mannequin reboot you've all been waiting for!
Experts still aren't sure, but they believe the whole reason her corpse managed to stay beautiful for this long is that it was buried so thoroughly that every method of decomposition basically went "lol j/k fuck that noise." Her tomb was located twelve meters underground, whilst her casket was placed Matryoshka-style inside seven other caskets and yet another burial vault. As for the body, it was wrapped in 20 layers in silk and suspended in a magnesium-infused acid bath. As a result, any corpse-eating bacteria would have suffocated through lack of oxygen and sheer fucking exhaustion.
Meanwhile, you haven't cleaned your shower curtain during Obama's second administration.
All kidding aside, Xin Zhui is one of the most important and awesome archaeological discoveries of the past century, as evidenced by the fact that she's still being studied to learn about ancient methods of corpse preservation. It'd go a lot faster if she didn't keep rising at night and absorbing the scientists' life force a la The Mummy, though.