In Master Format 2014, certain divisions (such as 15 to 20) are not assigned because:

The reasoning behind the choice being correct is rooted in the purpose of the MasterFormat system itself. By leaving certain divisions unassigned, the format maintains flexibility for future expansion. This approach allows for the inclusion of new technologies, practices, or categories that may emerge over time without necessitating a complete overhaul of the existing numbering and organization. Such foresight is crucial in the architecture, engineering, and construction industries, where innovation can rapidly change the landscape of project management and documentation.

In contrast, the other options suggest more restrictive or negative reasons for the lack of assignments. They imply that the unassigned divisions are no longer relevant or useful, which overlooks the proactive strategy of the MasterFormat to accommodate growth and change over time.