Why is the plateau area a restricted area for air force helicopters?

The air on the plateau is thin and low in oxygen content. Thin air needs the engine to provide more power to increase the reaction force to the air, so as to achieve the lift that can be achieved in plain areas. However, improving engine power requires more air intake, which is not available in plateau areas, so engine technology is the most critical! The civilian version of Black Hawk helicopter imported in 1980s was the most frequently used in the rescue of Wenchuan earthquake in 2008. It's not that we don't want domestic helicopters, but that we can't use them at high altitudes. Several Soviet helicopters also played a big role! !