Here's why:

* Extreme cold: Polar deserts experience extremely low temperatures year-round, often below freezing. This makes it difficult for most plants to survive.

* Low precipitation: Precipitation is very scarce in polar deserts, further limiting the availability of water for plants.

* Permafrost: The ground is permanently frozen, making it hard for plants to establish roots.

While some lichens and mosses can survive in these harsh conditions, the overall vegetation is extremely sparse.