Because it dosn't have good resources of food and water... and it is really hot and hard to live there. :)
Deserts are dry, no matter hot or cold. It is the lack of water that keeps population low.
There are both cold deserts as well as hot deserts.
The two main deserts I assume you are talking about is hot and cold deserts. The difference between them is temperature (warm moderate summers for hot deserts, low rainfall.) Cold deserts may have snowfall and much lower temperatures in the winter. Summers in cold deserts may get quite hot, however.
The Mojave is a low, subtropical desert while the Colorado Plateau Desert is a high, cold winter desert.
Most deserts have low populations densities.
Cold deserts (also called cold winter deserts) may get quite hot during the summer but may have very cold winters - examples are the Gobi Desert and the Great Basin Desert. Some cold deserts stay quite cool or even cold the year around - Antarctica and the Atacama Desert. Hot deserts generally stay quite mild during winter months - examples are the Sahara and Sonoran Desert.
There are hot subtropical deserts, cold winter deserts and cool coastal deserts so you need to specify a particular desert location.
cold
You would expect to find a cold desert biome in a region with low temperatures and low precipitation. Cold deserts typically have sparse vegetation, rocky terrain, and are characterized by cold winters and hot summers. Examples of cold deserts include the Gobi Desert in Asia and the Atacama Desert in South America.
There are two major classes of deserts:Hot Deserts such as the Sahara, the Arabian Desert and the Mojave Desert.Cold Deserts such as Antarctica, the Gobi Desert and the Patagonian Desert.
Because it is cold
There are hot deserts, such as the Sahara, Mojave and Kalahari and there are cold deserts such as Antarctica, the Gobi and the Patagonian Deserts.