Why doesn't the Minimal image support UEFI?
The Ubuntu help only stated it lacks them:
Note: While the minimal iso image is handy, it isn't useful for installing on UEFI-based systems that you want to run in UEFI mode. The mini iso lacks the proper files for booting the computer in UEFI mode. Thus, the computer will boot in BIOS compatibility mode, and the installation will be in BIOS mode.
And a bug report got triaged with a ....
This issue can be fixed by copying the whole 'EFI' folder of http://cdimage.ubuntu.com/daily-live/current/wily-desktop-amd64.iso into the USB storage made by http://archive.ubuntu.com/ubuntu/dists/wily/main/installer-amd64/current/images/netboot/mini.iso.
Or use this how to.
... but there is no actual source stating why it lacks (U)EFI. I have not seen anyone claim it is due to a copyright issue (that was my 1st assumption). At the moment I would state: because nobody cared about adding it.