Executives are frustrated with the delay between a system's concept and implementation. It gets back to looking creatively at ways to partition the project into smaller, more manageable pieces. Often you can deliver some very useful part of the system very early if you break the project down correctly. Those of us who are experienced in large systems have learned that a lot of money can go down the drain with a multi-year implementation plan that doesn't deliver anything in the interim. Users begin to lose confidence that they're going to get the system that meets their needs.
Requirements change as the industry or market changes.
On the other hand, users build a confidence level that you can do the job when things start to arrive in phases, as promised. Gilb talks about an evolutionary application delivery, which involves having the user determine the most valuable functionality first, and delivering that, and following on with everything else in successive waves. But just getting an organization prepared to take advantage of these approaches can take months. Ideally, IT managers would like to identify a troubled project before it gets too far off track. One thing to look for is a lot of things getting worked on and not many things getting completed.