To be honest, it does not take a year to produce a film and edit it. This is Summit's plan of trying to milk 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, fans 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 year later. Splitting the movie adaption into two movies might serve the story justice (we'll see), but releasing them a year apart is unwise.
I think they decided on doing them a year apart instead of 6 months because there are supposed to be a bunch of blockbusters next summer. I'm not sure exactly what, but that was what the official release said.