why DevOps transformations succeed where ITSM so often doesn't

I've consulted on many ITSM initiatives and it always seemd a struggle to effect improvement: all stick and no carrot; dragging horses to the river with no interest in drinking. My DevOps consulting these days is a different experience - of happy horses following willingly and drinking their fill. The difference between ITIL and DevOps is that DevOps works.

ITIL books always struck me as empty of people: a sterile cheerless place like a deserted hospital.
They start from process and metrics not humanity and emotion. I'm sure that ITIL authors and philosophers will disagree with me and say that ITIL speaks to culture change, but I don't feel that that is the reality which comes through in the books or in its application in organisations. ITSM has suffered from "binder-droppers" who push process rather than cultural change.

DevOps is very much a human-centric approach, in fact it is downright happy at times. It starts with cultural change, and sees practice and technology improvements flowing from that. It preaches trusting and empowering people and getting controls out of their way.

Likewise ITIL is in principle based around a service lifecycle, but in practice the books are about separate silos and every process is a separate silo optimised almost independently of the others. Therefore applying ITIL drives siloed thinking within an organisation, creating local optima which often degrade the overall value stream flow. Change management is everybody's favourite example of being good at managing risk and not very good at helping work get done. But many controls are applied in such a way as to make the control look effective yet not assist the value stream.

DevOps on the other hand is based on taking a holistic systems view, ensuring that optimisation is end to end and not in individual siloes, and that moving work down the value stream is top priority.

These are the two fundamental differences that makes DevOps transformations more successful than Service Management transformations have been in the past:
starting from people and culture;
and starting from systems and flow.