The cynical Internet trolls have already started, stating that the main reason for this was that its easier for lens flare. The real reason is experience. J.J. Abrams has said never shot with digital and has even commented in the past that he is comfortable with shooting on film.
Kathleen Kennedy has previously said that the new movie will be sensible with CGI and will use proper sets and models where its practical to use. Using film instead of digital could also indicate that the CGI is to be throttled back compared to other modern movies.