Why do projects fail? Why are business sponsors so often dissatisfied, even when projects complete on time and within budget? 

The reasons for project failure are numerous. They include misalignment with expected business benefits, misunderstanding of business problem, poor problem definition, poor stakeholder management, scope creeps, changing requirements, unavailability of critical resources, weak infrastructure or architecture, missing or delayed regulation (in regulated environments), poor project management, poor project planning and/or execution, bad choice of methodologies, etc.

Prince2, PMI, Agile/Scrum, Waterfall, DMADV, LEAN, DFSS, DMAIC, etc. The literature is filled with Project Management Methodologies that all claim to be the nirvana of project management, the one and only methodology that will ensure your project’s success, keep your customers happy, and minimize project costs. But are they? Really? 

The truth is that there is no panacea and no "one size fits all" methodology. They all have weaknesses and may not be suited to your specific business context.