He Wrote It in the Language People Actually Spoke

He was a Sanskrit scholar of the first rank. He wrote his masterpiece in a village dialect on purpose, and the scholars of Varanasi never forgave him for it.

The decision

Tulsidas began the Ramcharitmanas at Ayodhya in 1574, on Rama Navami.

He worked on it for something over two years. Continuing at Varanasi and Chitrakoot.

He could have written it in Sanskrit. He was entirely capable of it, and it was what a man of his learning was expected to do.

He wrote it in Awadhi, the spoken language of the eastern Gangetic plain, the language of farmers and traders and women at home.

His reason was not obscure and he stated it. Sanskrit was known to a small number of people. The story of Rama was for everyone. So it would be written in the language everyone had.