From my understanding, there are several reasons this one has taken so long. One is that the game was originally going to be made as one game. Then the team realized how long it was taking and how much more time it would take, so during one of the birthday bashes, they announced they had decided to break it into three installments. Then, another, is trying to move what's written into something that works on the computer. And, from my understanding, there have been several things that have had to be built from scratch. Originally, the team hoped to release Part 1 one year, Part 2 the following, and Part 3 after that. In other words, kinda like Lord of the Rings, where a new movie was released every year.
I hope that if this doesn't answer your question, it helps to clarify...at least the original intent of the team
