A concise explanation for why large-scale software projects frequently fail, from Michael Krigsman:

Why do projects fail? Very often, the roots of failure lie in non-technical areas related to project management, organizational politics, and lack of consensus across stakeholders. In plain English, failures often occur when people with conflicting agendas can’t put aside their own narrow concerns and agree on a course of action that is best for the project as a whole.

In our business, we encounter a lot of people who specialize in Change Management, Organizational Development, Leadership Alignment, and similar fine pursuits. Few of them, however, can capture software implementation dynamics as succinctly as someone (Krigsman) who has clearly lived through these projects on the front lines.