Through the years, I have had many conversations on the topic of why projects fail. One of the best lists I've come across is from the UK OGC Best Practices site - with some slight edits for non-governmental domains.
Inadequate linkage between the project and the broader organization's key strategic priorities, including agreed-to measures of success.
Insufficient involvement by senior management and business ownership and leadership.
Ineffective engagement with stakeholders.
Limited experience with effective project management and risk management approaches.
Too little attention to elaborating development and implementation strategies into manageable steps.
Premature evaluation of proposals with a focus on initial price rather than long-term value (especially with respect to securing delivery of needed business benefits).
Ineffective communications between the suppliers of technology across the projects.
Lack of effective project team integration between clients, the supplier team, and the primary responsible organization.