Why did Charles Dickens write about grim and rather miserable places?

That was his Universe. He was the urban writer par Excellence in an era when all English writers set their works either in the countryside, or in small market or industrial towns, or in exotic places throughout the British Empire. This was the Word he knew and UNDERSTOOD. He preferred to write what he knew best, and he had the courage to describe small universes the well-to-do preferred to ignore (such as the East End).