Why is Azerbaijani not classified as a Caucasian language?

Azerbaijani is not classified as a Caucasian language because it belongs to the Turkic languages family, specifically the southwestern (Oghuz) branch.12

While the term "Caucasian languages" refers to groups indigenous to the Caucasus region—such as the Northwest Caucasian, Northeast Caucasian, and South Caucasian families—Azerbaijani is considered an intrusive language that was introduced to the area during historical times. Its presence in the region is the result of centuries of migration and the Turkicization of the local population, which began with the arrival of the Seljuq Turks in the 11th century. Consequently, it is linguistically distinct from the indigenous Caucasian language families.12