I'm a commercial general contractor, so I can probably give you a good answer, but I need to make sure I understand your question first. I'm not quite sure if you're asking why construction projects, once begun, are slow to be completed, or if you are you asking why Disney is so slow to decide to build/begin a project in the first place.
The answer to the first question is that construction of a commercial structure (that is, a building other than a house) is very, very complicated and takes countless hours of planning, designing, and checking by registered architects, engineers, and other professionals. Every footing, every wall, etc. has to be laid out with surveying and measuring equipment. It's easy to walk into a building and not wonder why it doesn't fall down around you, how it holds up to central Florida hurricane season, and how all the corners and angles are perfectly straight and everything lines up so nicely. These are things the general public takes for granted, but are very time consuming and expensive to pull off. Most people don't think about any of this until they see a pile of rubble on the news.
There is a process of building a building that can't be shortcut, even if you have unlimited manpower and resources. On this very subject, a guy I know told me once "One woman can have a baby in nine months. Nine women can't have a baby in one month." At each major step along the way, construction has to stop for architects, engineers, contractor personnel, building inspectors, and the owner of the building to inspect each element before it's concealed to ensure it was done properly. It would be pretty tough to work on the footings under Cinderella Castle right now, so everything is checked along the way. In our industry, failure to properly build and inspect some small thing early in the process has lead to countless failures and deaths.
There is also the problem of lead times and delivery of critical materials. If one person doesn't live up to a promise of delivering a critical part, or if something is fabricated incorrectly, the project can be delayed for weeks. Then of course, there's the weather...
This post has almost convinced me to find a new career.