Question

Why do novels use vernacular

Answer 1

Vernacular is the language spoken by common people. Naturally novels written in vernacular had great appeal to common people and were widely read.
By coming closer to different spoken languages of the people, these novels produce a sense of a shared world between diverse people in a nation and inculcate a sense of belonging to the nation

Answer 2

here is your answer

Vernacular literature is literature written in the vernacular—the speech of the "common people".

In the European tradition, this effectively means literature not written in Latin. In this context, vernacular literature appeared during the Middle Ages at different periods in the various countries; the earliest European vernacular literatures are Irish literature, Welsh literature, Anglo-Saxon literature and Gothic literature