Why did ankylosaurus live in the desert?

Ankylosaurus didn't live in the desert. They lived on the coastal plains of western North America between 66.5 and 65.5 million years ago. The environment was warm, humid, and subtropical, with wet and dry seasons.

However, some other ankylosaurs, which were relatives of Ankylosaurus, did live in the Gobi Desert of Asia. Why they lived there is difficult to say. It's similar to asking why camels live in the desert.