Because the Teracube is on a build type for which Android 15 is available, and the Fairphone 3 is on a build type for which it is not (yet).
https://images.ecloud.global/official/zirconia/ / https://images.ecloud.global/official/emerald/
(There are two different Teracube 2e, codenamed zirconia and emerald, but it’s not important for answering your question.)
https://images.ecloud.global/official/FP3/
(And I worded it this way because an FP3 currently can be on a build type for which Android 15 is available already … https://images.ecloud.global/community/FP3/ … but swapping to “community” from “official” is a new install with a data wipe, “community” doesn’t support relocking the bootloader, and once the “official” build type caught up to Android 15, the “community” build type will be phased out on FP3 and users will need to use “official” anyway.)
Also … TerraCube on Steam … https://terracubes.net/ … https://www.terracube.com/ … … … https://myteracube.com/