Legendary director Quentin Tarantino is known to write characters with particular actors/actresses in mind for his films; he envisioned Jennifer Lawrence as the actress to star in his upcoming film The Hateful 8, but she said no due to having other commitments with films like Joy, and the publicity for the Hunger Games movies.
Read the following quote for more:
“She was just doing me a courtesy to see me, I think. She was doing Joy. She had to [do] all this publicity on the Hunger Games movies. There was just no way in the world that she was available. Having said that, I’m glad I didn’t cast somebody that young. I think I absolutely positively made the right choice, as far as the ages of the characters.” (Quentin Tarantino)