From Variety:

Kristen Stewart was attached to also star in the pic but when Smith became engaged in the project, Stewart fell off with the feeling that the age difference between the two would be too large a gap

From THR:

Kristen Stewart is no longer set to star in Focus, a Warner Bros. heist project being directed by Crazy, Stupid, Love helmers Glenn Ficarra and John Requa. And as Stewart exits, the project could be landing a male star in Will Smith.