Here's why:

* Vastness: The Mongolian steppe covers approximately 70% of the country, making it the largest continuous steppe in the world.

* Climate: Mongolia's climate is characterized by cold, dry winters and short, warm summers. This harsh climate is ideal for the development of steppe ecosystems.

* Flora: The steppe is characterized by short, drought-resistant grasses, herbs, and shrubs. There are also some scattered trees, particularly along rivers and in protected areas.

* Fauna: The Mongolian steppe is home to a variety of animals adapted to the harsh environment, including wild horses, gazelles, wolves, and birds of prey.

While other ecosystems like forests, mountains, and deserts exist in Mongolia, they are significantly smaller than the vast expanse of the steppe.