Why are Paleo-Siberian languages sometimes described as remnants of older Siberian language diversity?

Paleo-Siberian languages are described as remnants of older Siberian language diversity because they represent the surviving fragments of languages that were once spoken over much broader territories before being displaced or absorbed by more recent, expansive language groups.12

Historically, these languages were encroached upon by the spread of Uralic speakers from the Sayan Mountains in the west and Tungusic speakers from the Transbaikal region in the east. Evidence suggests that groups such as the Yeniseian, Luorawetlan, and Yukaghir occupied significantly wider areas as recently as the 17th century.12

Many other languages once present in the region were likely lost entirely, having been "swallowed up" by culturally vigorous intruders like the Sakha and various Tungus tribes. Consequently, the four distinct Paleo-Siberian families—which are not genetically related to one another—are viewed as the last vestiges of a much greater linguistic diversity that characterized prehistoric Siberia.23