Juanita was mummified because she was frozen. On Mount Ampato (location of her death) is almost always covered in ice and snow. She was stuck in the snow for over 500 years, perfectly preserving her. While a normal body would decay, hers did not, and this is why...
For a person to be mummified, there must be no decaying causing bacteria or fungi. In the location that she was in, it was to cold for any bacteria or fungi to grow, leaving the body to preserve itself.