In some environments, users have an isolated vCenter server and are required to manage the cluster using a custom image.
Using an isolated vCenter server where the LCM depot is unable to sync with the Broadcom website, requiring the image to be downloaded manually.
The image will not appear in the dropdown unless the ESXi offline bundle has been uploaded to the Lifecycle Manager depot.
Steps to Upload the ESXi Offline Bundle ZIP File to the LCM Depot -
For a custom ISO, you can directly upload it, and it will display the included Vendor Addons. Alternatively, you can upload the standard image and manually add the hardware vendor addons.
Reference -