The location was deliberately obscured.
An often repeated account maintains that the soldiers in his [Genghis Khan's] funeral cortege killed every person and animal encountered on the forty-day journey, and that after the secret burial, eight hundred horsemen trampled repeatedly over the area to obscure the location of the grave.
Then, according to these imaginative accounts, the horsemen were, in turn, killed by yet another set of soldiers so that they could not report the location of the site; and then, in turn, those soldiers were slain by yet another set of warriors.
from Jack Weatherford's 'Genghis Khan and the Making of the Modern World'
https://www.amazon.com/dp/1491513705/ref=cm_sw_r_tw_dp_x_-ZKfAbM3YRVMJ
photo: The Genghis Khan Statue Complex in Mongolia