They fail because the executive team is sleeping. Anyone home?
They fail because the executive team is busy being buys with executive stuff.
They fail because because a lack of planning. The plan sucks!
They fail because the project was a bad idea to start with in the first place.
They fail because nobody took the time to clearly articulate and write down ALL the requirements.
They fail because the project manager and planners suffered from the planning fallacy syndrome.
They fail because ...
 
to be continued!

Below is an excerpt of an answer Erik Hamburger of Ambidexter Management answered on LinkedIn. Click here for the full question and all the answers.

I've done Prince2 Practioner, MSP Practioner, and prepared for IPMA B.

The primary reason for Prince2 was that at that time (1997) it had just emerged from the UK into the Netherlands and it seemed (and proved) to be a very powerful process driven PM methodology. I has studied the PMI PMBoK at that time and found (and still do) that more descriptive and Prince2 more prescriptive.

PMBoK and Prince2 can and should be used in conjunction. More about PMI a bit later.

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