Why did Braj Bhasha and Awadhi become important literary languages?

Braj Bhasha and Awadhi became prominent literary languages primarily due to their deep roots in the devotional movements of the medieval period and their ability to serve as vehicles for religious expression.12

Braj Bhasha
Braj Bhasha developed a robust literary tradition, particularly through the bhakti (devotional) poetry of the early medieval period (c. 1450–1700 CE) and the erotic poetry of the late medieval riti kal (c. 1700–1850 CE). Its importance was bolstered by its strong foundation in folk literature and songs, particularly those centered on the worship of the deity Krishna. Poets such as Sūrdās and Mīrā Bāī utilized the language to create works that achieved wide acceptability, allowing Braj to persist as a primary medium for poetry until the late 19th century.134

Awadhi
Awadhi gained significance as a major literary dialect of Eastern Hindi, most notably through the work of Tulsīdās. His epic, the Rāmcaritmānas, became a foundational sacred text for the Hindi-speaking region. Additionally, Awadhi was used by Muslim writers such as Jāyasī, whose epic Padmāvat demonstrated the language's capacity to adhere to the conventions of Sanskrit poetics.243