To be honest, it does not take a 年 to produce a film and 編集 it. This is Summit's plan of trying to ミルク the Twilight Saga of what it's worth. However, this isn't the best plan, considering, the momentum of the series is slowing down. I mean, look at the Twilight series spot now. All the fansites. Hardly any news, therefore, ファン become uninterested until things pick up again.
So in answer to your question, there isn't rally a good reason why they are releasing part 2 a 年 later. Splitting the movie adaption into two 映画 might serve the story justice (we'll see), but releasing them a 年 apart is unwise.
The book was too long to make into one movie...they would have had to cut most of the story out....two movies...more of the story and もっと見る money for Summitt ....lol
I think they decided on doing them a 年 apart instead of 6 months because there are supposed to be a bunch of blockbusters 次 summer. I'm not sure exactly what, but that was what the official release said.