Obviously Disney takes a long time because like Everest the send teams of people to study that area and the Mt to get an exact replica of it. I am sure down the artifacts that are on display when you walk through to get there are exact replicas of items the saw or bought.
I also think, they like to open new attractions each year so they can get repeat business. Obviously if they opened up Stitch, Toy Story, Everest, Nemo, Pirates (remodeled) all in one year and then it took them 2 to 5 years to open 5 more new attractions the repeat business would probably go down. People would wait and hit them all when the new attractions opened.
I have to agree about the post regarding Six Flags, we have one about 15 mins from us and of course we have seasons passes, but they can't hold a candle to Disney. From the atmosphere, employees, food, shows, rides, cleaniness. Six Flags will never be up to Disney's standards. Yes, it may take them 6 mos to put up a ride, but it is exactly that a ride, a steel coaster on a slab of concrete.
That is just my opinion on this subject.