According to media reports, White had to revoke his commitment to the score due to scheduling conflicts. A statement posted by Disney says:
Oscar winner Hans Zimmer, the musical mastermind behind Disney’s and Jerry Bruckheimer Films’ ‘Pirates of the Caribbean,’ has signed on to compose the score for ‘The Lone Ranger.’ Jack White, who had originally been contemplated to score and has contributed several pieces of music to the production, was logistically unavailable due to scheduling conflicts that arose when the film’s release moved to July of 2013.
Reports do not go into detail about the nature of White’s scheduling conflicts, so we aren’t sure what projects he has that conflict with Disney’s film.
The Lone Ranger is slated to debut in theaters July 3, 2013.