I say projects mostly fail because of management. Management has a centric role in project success or project failure. Here's how:
- Management support: If management supports the project (and the project manager); if executives care about the project, if they follow up its issues/its needs then the project will most likely be a success. On the other hand, if most of the executives don't care about the project, or maybe they want it to fail because of hidden agendas, then the project is definitely doomed. And I'm saying definitely because the project manager wants management on board for his project to succeed, it's not the project manager who owns the success (or the failure) or the project, it's someone from management. If management is not on board, then here's what may happen: Project funds would be blocked, unrealistic deadlines will be imposed, impossible change requests would be approved, etc...
- Management control: Executives must control the project from a higher level, they must ensure that it's meeting its targets when it comes to deliverables, and that it remains at all time on schedule, on scope, on budget. Additionally, management, through Project Portfolio Management and the PMO, must always ensure that the project is aligned with the company's business objectives and that correct project management is used. If this control does not exist, then the project is at the mercy of the project manager, his skills, and his devotion to his work, which is (nearly always) not enough.
You see without management's support, involvement, and control of the project, a super project manager will be the only hope for the project to succeed, but even a super project manager cannot make a project succeed if management does not care about the project, and nor its outcome. Without management support, the project manager will be left (alone) at the mercy of the client, the project team, and possibly all the other stakeholders (including management!).