Why did Sanskrit become the primary literary language

during the Gupta period?

It is important to understand that Sanskrit is not a national language. This is the language of the cultural environment. Initially, Sanskrit was used as the common language of the priestly class, while the ruling classes preferred to speak Prakrit. Sanskrit finally becomes the language of the ruling classes already in late antiquity in the Gupta era (4th - 6th centuries AD), which can be judged by the complete crowding out of prakrit from cave art in this era, as well as by the flourishing of Sanskrit literature. Thus, it is believed that the bulk of Sanskrit literature was written precisely in the era of the Gupta Empire.