There are two reason why they are being released a year apart. The honest reason, post-production. Even though they completed the vast majority of photography in the innitial 18 months, that didn't include the editing, score, and perhaps most importantly (and time consuming) the special effects. Having a year apart allows them to edit at thier leisure. So you are saying they could have edited everything at the same time? Then we'd have gotten FotR much, much later. Probably a year from now.
The second reason, and probably the real reason, is money. This strategy allows New Line to milk every last dollar out of it's customers. Were all three movies showing at around the same time, they'd be competing with eachother. Each movie would be stealing potential customers from the other movies. For example: What really makes the big $ for movies is repeat viewings. How many repeat viewings would FotR get if movie-goers knew TTT was out at the same time? Not many. The way it is now New Line can release one product at a time, banking on fans anxiousness as thier marketing tool. An excellent example is the DVD marketing scheme. By releasing the products so far apart they are getting many fans to buy both. The point is that they don't want thier own products to compete with eachother, especially when those products have the same name. You'll notice that studios tend to release their big name DVD's at intervals. They want to milk as much money from the same customer as possible.
