Which is observant... but ...
- Fallout 1 release: 1997
- Games Workshop Release Adeptus Mechanicus: 2016
- GW publish 'Space Marines' as we understand them (2nd ed 40k): 1993
- GW expands and grows the background details of Imperium (3rd ed+): 1998
In the late 90's Games Workshop was still growing and the modern concepts you've named didn't exist except as a glint in the eyes of Gav Thorpe et al. Even the 'early' development art of the earlier '1st edition' space marines was conceptually different from what we see today.
It is, of course, sensible to liken BoS to Space Marines, but the heavily pseudo-medieval imperium design had yet to become concreted in place, that only really grew into place after GW released 3rd edition (1998) and so it would have been impossible for that later design to influence Black Isle. Before the 3rd ed the Marines were very Neo-Gothic and, if I can add my opinion lacked a lot of the deeper styling.
Further to add into this mix is that GW in the 90's was a flourishing company which likely had not yet had dramatic enough an impact into 'pop-nerd-culture' and I'd have to question if Black Isle or the Devs of FO1 even knew of the existence of GW (citation require on this... does anyone know more?)
While the above is purely my own speculation, I like to think that, based on the details I know of both franchises, I hope I'm able to piece together something close to the truth.