Why did Western writers call Xinjiang Chinese Turkistan?

Western writers historically used the term "Chinese Turkistan" to refer to the region now known as the Xinjiang Uyghur Autonomous Region primarily to distinguish it from "Russian Turkistan".1

The term "Turkistan" was used by 19th-century explorers and geographers to describe the broader regions of Central Asia inhabited by Turkic peoples. Because this vast area was divided by the Pamirs and Tien Shan mountain systems into different spheres of influence, the labels "West Turkistan" (under Russian control) and "East Turkistan" (under Chinese control) were adopted to provide geographical clarity.23

While the region has been known to the Chinese as Xiyu ("Western Regions") for centuries and became Xinjiang ("New Borders") following its annexation by the Qing dynasty in the 18th century, Western writers maintained the "Chinese Turkistan" nomenclature to categorize the territory within the context of their own historical and political frameworks of the time.21