For the DN NUCs, the BIOS is unique to the processor present in the NUC. As a result, when you do a search without distinguishing which processor, you will not see a BIOS entry in the list.
Here are links to the BIOS file(s) based upon processor:
Hope this helps,
My link was to the "Downloads for Intel® NUC Kit with 7th Generation Intel® Core™ Processors"
It used to have BIOS in it. It seems they have now changed to device specific download pages.
Found the new link here on search.