After visiting the official documentation of CKAN, I was unable to find why are we using a PostgreSQL database in CKAN from its inception.
Is there any way to use another database with CKAN?
No, the only currently supported database backend is PostgreSQL. It was chosen as it is an open source, well supported and widely available solution. Support for other backends would need to be added to the core library as new feature (CKAN uses SQLAlchemy as ORM, which would help). If someone is up for it they can check the contributing docs and get in touch with the maintainers.