Which reason describes why ITIL is so successful?

The ITIL volumes are comprehensive and detailed, not concise - they cover a broad range of service management practices across multiple books.

ITIL is a framework of best practices rather than a prescriptive, vendor-specific toolset. Because it is not tied to any particular vendor platform or product, organizations using any combination of technology can adopt and adapt it, which is the primary driver of its global widespread adoption.

ITIL provides guidance and best practices but does not prescribe exact steps for success; it must be tailored to each organization's context.

ITIL is a service management framework, not a project management methodology - that role is filled by frameworks such as PRINCE2 or PMBoK.