Because the deadline is only a target, users will negotiate additional facilities with equanimity, considering full functionality to be the more important deliverable. Certainly not the consultants who help the users realize their expectations. Further, it must be admitted that most project managers will reluctantly accept change to the scope or functionality of a system if the users press hard enough. The end result is a conspiracy between the users and IT management. The original system as contracted expands to include further bells and whistles that both sides failed to see in the beginning.
Guilt contributes to the fiasco.
Having failed themselves to specify properly everything that was wanted, neither users nor managers are eager to provide an honest critique of the project. This is where the charade starts. Everyone knows that the system is growing even as dates are slipping. Reality check If you have a deadline and you really mean to keep it, build a realistic schedule that uses back-projection from the final date. Publish this schedule and publicly decrement it every day.