Why are Chuvash and Common Turkic not mutually intelligible?

The lack of mutual intelligibility between Chuvash and Common Turkic languages stems from their early historical divergence into two distinct types: Bolgar Turkic and Common Turkic.1

Chuvash is the only modern descendant of the extinct Volga-Bolgarian language, which belonged to the Bolgar Turkic branch. This branch is characterized by archaic features and specific phonetic differences that set it apart from the rest of the Turkic language family. Key phonetic distinctions include regular representations such as r versus z and l versus š—for example, the Chuvash word śer ("hundred") compared to the Turkish yüz, or śul ("age") compared to the Turkish yaş.21

Because Chuvash evolved separately from the Common Turkic group, it constitutes a distinct branch that differs considerably from other Turkic languages, resulting in the absence of mutual intelligibility.21