Quoted from Rarehero:
Multiple games for customers to chose from is good for business, not bad. Anyone who wants Hobbit will buy Hobbit. Someone who has no interest in Hobbit might want another choice. Stern has like 10 games in production. No game is derailing the other.
Yes. But the hobbit is still relatively new. And until the JJP 3 is close to shipping and the hobbits preorders are close to being complete it wouldn't be good business for a reveal now. Some people would cancel there hobbit order for it, and that doesn't mean more money for JJP.
Same reason why stern hasn't announced their first LCD game yet, a big bulk of GB preorders would disappear.
Jack said in an interview that they might announce a couple months before shipping because they need to get an understanding of the interest for ordering parts etc