What may cause newly purchased iPhone devices to not be asked to enroll in Automated Device Enrollment?

The reason that the first option is correct relates to the requirements of Automated Device Enrollment (ADE). For an iPhone to automatically enroll in a Mobile Device Management (MDM) solution upon setup, it must be assigned to a default MDM server. This assignment is done in Apple's Business Manager or School Manager, which essentially links the device to the MDM solution.

If a device lacks a default MDM server assignment, it will not recognize where to connect for enrollment, which means the user will not be prompted for enrollment into the MDM solution during the setup process. This fundamental aspect of ADE ensures that devices can be centrally managed as intended.

In contrast, while other reasons like not being connected to Wi-Fi, compatibility with MDM, or requiring manual enrollment could lead to issues during the setup or management process, they do not fundamentally disrupt the Automatic Device Enrollment flow like the lack of an MDM server assignment does. Without this specific assignment, the device simply won't know it should initiate enrollment, making this the key factor in the question.